Condensed Consolidated Statements of Comprehensive Income (Unaudited) (Parenthetical) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Statement Of Income And Comprehensive Income [Abstract]
Tax (provision) benefit of unrealized gains (losses) on derivative financial instruments	$ (289)	$ 505	$ 50	$ 1,342
Tax provision (benefit) on reclassification adjustment for (gains) losses on derivative financial instruments	234	(395)	13	(753)
Tax benefit of amortization of benefit plan costs	(198)	(245)	(395)	(518)
Tax provision of unrealized gains	(32)	37	(57)	(7)
Tax provision on reclassification adjustment for gain on investment	$ 3	$ 3	$ 15	$ 3