Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 200,274	$ 188,301	$ 190,509
Investment securities available for sale:
Net unrealized gains (losses)	(17,900)	(37,873)	49,038
Net unrealized gains on available for sale securities transferred to held to maturity securities	0	17,434	0
Amortization of net unrealized gains on available for sale securities transferred to held to maturity securities	(5,887)	(555)	0
Reclassification adjustment for net gains realized in net income	(9,316)	(8,133)	(494)
Income tax (expense) benefit	12,565	11,074	(18,636)
Other comprehensive income (loss) on investment securities available for sale	(20,538)	(18,053)	29,908
Defined benefit pension and postretirement obligations:
Prior service cost, net of amortization	(73)	50	58
Plan amendments	1,759	0	0
Net loss, net of amortization	(4,026)	(15,636)	(17,079)
Income tax benefit	815	5,873	6,507
Other comprehensive loss on pension and postretirement obligations	(1,525)	(9,713)	(10,514)
Total other comprehensive income (loss)	(22,063)	(27,766)	19,394
Comprehensive income	$ 178,211	$ 160,535	$ 209,903