Accumulated Other Comprehensive Income (Loss) - Reclassifications out of AOCI - (Details) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassifications out of accumulated other comprehensive income, net of tax
Interest expense		$ (17,014,000)	$ (8,317,000)	$ (10,328,000)
Other noninterest income		6,670,000	6,437,000	5,298,000
Securities gains, net		1,421,000	122,000
Other-than-temporary impairment on securities	$ 753,000	753,000		489,000
Provision for income taxes		(81,251,000)	(52,760,000)	(50,902,000)
Net income		87,554,000	101,282,000	99,473,000
Amounts reclassified from accumulated other comprehensive income (loss)		(319,000)	(664,000)	(1,123,000)
Amount Reclassified from Accumulated Other Comprehensive Loss
Reclassifications out of accumulated other comprehensive income, net of tax
Net income		319,000	664,000	1,123,000
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)		(170,000)	(170,000)	(189,000)
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		275,000	275,000	305,000
Provision for income taxes		(105,000)	(105,000)	(116,000)
Net income		170,000	170,000	189,000
Unrealized gains and losses on securities
Reclassifications out of accumulated other comprehensive income, net of tax
Amounts reclassified from accumulated other comprehensive income (loss)		413,000	75,000	(302,000)
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		(1,421,000)	(122,000)
Provision for income taxes		542,000	47,000
Net income		(879,000)	(75,000)
Other-than-temporary impairment losses on available for sale securities | Amount Reclassified from Accumulated Other Comprehensive Loss
Reclassifications out of accumulated other comprehensive income, net of tax
Other-than-temporary impairment on securities		(753,000)		(489,000)
Provision for income taxes		(287,000)		(187,000)
Net income		466,000		302,000
Amortization of defined benefit pension
Reclassifications out of accumulated other comprehensive income, net of tax
Amounts reclassified from accumulated other comprehensive income (loss)		(562,000)	(569,000)	(632,000)
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		908,000	920,000	1,021,000
Provision for income taxes		(346,000)	(351,000)	(389,000)
Net income		$ 562,000	$ 569,000	$ 632,000