Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 89,192	$ 57,983	$ 158,648	$ 114,253
Investment securities available for sale
Net unrealized gains (losses)	(18,919)	15,218	(60,754)	18,370
Net unrealized gain (loss) on available for sale securities transferred to held to maturity securities	0	(9,474)	0	(14,738)
Amortization of net unrealized gain (loss) on available for sale securities transferred to held to maturity securities	(335)	(1,548)	(632)	(2,575)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax	2,015	0	2,015	0
Reclassification from OCI due to change in accounting principle	0	0	(84)	0
Reclassification of certain tax effects from OCI	0	0	(8,419)	0
Income tax (expense) benefit	4,705	(1,601)	15,340	(406)
Other comprehensive income (loss) on investment securities available for sale	(12,533)	2,595	(52,533)	651
Defined benefit pension and postretirement obligations
Amortization of prior service cost	38	37	(76)	75
Amortization of actuarial loss (gains)	(465)	(487)	929	(975)
Reclassification of certain tax effects from OCI	0	0	(5,235)	0
Income tax (expense) benefit	109	171	(216)	342
Other comprehensive income (loss) on pension and postretirement obligations	318	279	(4,597)	558
Total other comprehensive income (loss)	12,215	(2,874)	57,130	(1,209)
Comprehensive income	$ 76,977	$ 60,857	$ 101,518	$ 115,462