Accumulated Other Comprehensive Income (Loss) - Reclassifications out of AOCI - (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassifications out of accumulated other comprehensive income, net of tax
Interest expense	$ (4,092)	$ (2,036)	$ (11,464)	$ (6,229)
Securities gains, net	1,278		1,388	122
Other-than-temporary impairment on securities			753
Provision for income taxes	(17,677)	(14,387)	(40,710)	(39,368)
Net income	35,046	28,095	85,133	77,105
Amount Reclassified from Accumulated Other Comprehensive Loss
Reclassifications out of accumulated other comprehensive income, net of tax
Net income	(640)	168	(370)	433
Gains on sales of available for sale securities
Reclassifications out of accumulated other comprehensive income, net of tax
Amounts reclassified from accumulated other comprehensive income (loss)	(35)	(42)	(140)	(129)
Gains on sales of available for sale securities | Amount Reclassified from Accumulated Other Comprehensive Loss | Interest rate contracts
Reclassifications out of accumulated other comprehensive income, net of tax
Interest expense	57	69	226	209
Provision for income taxes	(22)	(27)	(86)	(80)
Net income	35	42	140	129
Unrealized gains and losses on securities
Reclassifications out of accumulated other comprehensive income, net of tax
Amounts reclassified from accumulated other comprehensive income (loss)	325		393	75
Unrealized gains and losses on securities | Amount Reclassified from Accumulated Other Comprehensive Loss
Reclassifications out of accumulated other comprehensive income, net of tax
Securities gains, net	(1,278)		(1,388)	(122)
Provision for income taxes	487		529	47
Net income	(791)		(859)	(75)
Other-than-temporary impairment losses on available for sale securities
Reclassifications out of accumulated other comprehensive income, net of tax
Other-than-temporary impairment on securities	(753)		(753)
Provision for income taxes	(287)		(287)
Net income	466		466
Amortization of defined benefit pension
Reclassifications out of accumulated other comprehensive income, net of tax
Amounts reclassified from accumulated other comprehensive income (loss)	(116)	(126)	(349)	(379)
Amortization of defined benefit pension | Amount Reclassified from Accumulated Other Comprehensive Loss
Reclassifications out of accumulated other comprehensive income, net of tax
Salaries and employee benefits	188	204	564	612
Provision for income taxes	(72)	(78)	(215)	(233)
Net income	$ 116	$ 126	$ 349	$ 379