Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$ 215,632	$ 149,296	$ 131,461
Other comprehensive income (loss) before income taxes:
Net change in unrealized loss on available for sale securities and hedges	(425)	(57,346)	(21,270)
Reclassification of net losses realized and included in earnings	5,801	4,016	3,010
Valuation adjustment for pension plan amendment	17,315
Other valuation adjustments for employee benefit plans	(10,929)	(12,748)	(33,971)
Amortization of unrealized net loss on securities transferred to held to maturity	3,786	3,830	3,530
Other comprehensive income (loss) before income taxes	15,548	(62,248)	(48,701)
Income tax expense (benefit)	4,088	(22,311)	(18,180)
Other comprehensive (income) loss net of income taxes	11,460	(39,937)	(30,521)
Comprehensive income	$ 227,092	$ 109,359	$ 100,940