Annual report pursuant to Section 13 and 15(d)

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical)

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CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]      
Unrealized gain due to fair value adjustments on interest rate swap agreements, taxes $ 0 $ 138 $ 1,562
Unrealized loss due to fair value adjustments on available-for-sale securities, taxes $ 0 $ 0 $ 572