Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 49,091	$ 49,400	$ 91,625	$ 96,072
Investment securities available for sale:
Net unrealized gains (losses)	22,321	(35,224)	82,743	12,194
Amortization of net unrealized gains on available for sale securities transferred to held to maturity securities	(1,452)	0	(3,024)	0
Reclassification adjustment for net gains realized in net income	(3,116)	(1,242)	(6,214)	(1,242)
Income tax (expense) benefit	(6,773)	13,923	(28,048)	(4,182)
Other comprehensive income (loss) on investment securities available for sale	10,980	(22,543)	45,457	6,770
Defined benefit pension and postretirement obligations:
Prior service cost, net of amortization	12	12	25	25
Amortization of actuarial losses	483	533	965	1,065
Income tax expense	(189)	(208)	(378)	(416)
Other comprehensive income on pension and postretirement obligations	306	337	612	674
Total other comprehensive income (loss)	11,286	(22,206)	46,069	7,444
Comprehensive income	$ 60,377	$ 27,194	$ 137,694	$ 103,516