Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Net income	$ 188,301	$ 190,509	$ 188,692
Investment securities available for sale:
Net unrealized gains (losses)	(37,873)	49,038	(158,207)
Net unrealized gains on available for sale securities transferred to held to maturity securities	16,879	0	0
Reclassification adjustment for net gains realized in net income	(8,133)	(494)	(564)
Income tax (expense) benefit	11,074	(18,636)	61,266
Other comprehensive income (loss) on investment securities available for sale	(18,053)	29,908	(97,505)
Defined benefit pension and postretirement obligations:
Prior service cost, net of amortization	50	58	72
Net gain (loss), net of amortization	(15,636)	(17,079)	40,148
Income tax (expense) benefit	5,873	6,507	(15,562)
Other comprehensive income (loss) on pension and postretirement obligations	(9,713)	(10,514)	24,658
Total other comprehensive income (loss)	(27,766)	19,394	(72,847)
Comprehensive income	$ 160,535	$ 209,903	$ 115,845