Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 57,983	$ 49,091	$ 114,253	$ 91,625
Investment securities available for sale:
Net unrealized gains	15,218	22,321	18,370	82,743
Net unrealized loss on available for sale securities transferred to held to maturity securities	(9,474)	0	(14,738)	0
Amortization of net unrealized gains on available for sale securities transferred to held to maturity securities	(1,548)	(1,452)	(2,575)	(3,024)
Reclassification adjustment for net gains realized in net income	0	(3,116)	0	(6,214)
Income tax expense	(1,601)	(6,773)	(406)	(28,048)
Other comprehensive income on investment securities available for sale	2,595	10,980	651	45,457
Defined benefit pension and postretirement obligations:
Amortization of prior service cost	(37)	12	(75)	25
Amortization of actuarial losses	487	483	975	965
Income tax expense	(171)	(189)	(342)	(378)
Other comprehensive income on pension and postretirement obligations	279	306	558	612
Total other comprehensive income	2,874	11,286	1,209	46,069
Comprehensive income	$ 60,857	$ 60,377	$ 115,462	$ 137,694