Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 42,534	$ 46,672
Investment securities available for sale:
Net unrealized gains (losses)	60,422	47,418
Amortization of net unrealized gains on available for sale securities transferred to held to maturity securities	(1,572)	0
Reclassification adjustment for net gains realized in net income	(3,098)	0
Income tax (expense) benefit	(21,275)	(18,105)
Other comprehensive income (loss) on investment securities available for sale	34,477	29,313
Defined benefit pension and postretirement obligations:
Prior service cost, net of amortization	13	13
Amortization of actuarial losses	482	532
Income tax (expense) benefit	(189)	(208)
Other comprehensive income (loss) on pension and postretirement obligations	306	337
Comprehensive income	77,317	76,322
Total other comprehensive income (loss)	$ 34,783	$ 29,650