Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 56,270	$ 42,534
Investment securities available for sale:
Net unrealized gains	3,152	60,422
Net unrealized losses on available for sale securities transferred to held to maturity securities	(5,264)	0
Amortization of net unrealized gains on available for sale securities transferred to held to maturity securities	(1,027)	(1,572)
Reclassification adjustment for net gains realized in net income	0	(3,098)
Income tax (expense) benefit	1,195	(21,275)
Other comprehensive income (loss) on investment securities available for sale	(1,944)	34,477
Defined benefit pension and postretirement obligations:
Amortization of prior service cost	(38)	13
Amortization of actuarial losses	488	482
Income tax expense	(171)	(189)
Other comprehensive income (loss) on pension and postretirement obligations	279	306
Total other comprehensive income (loss)	(1,665)	34,783
Comprehensive income	$ 54,605	$ 77,317