Investment Securities - Unrealized Losses and Fair Value Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value
Less than Twelve Months	$ 840,488	$ 489,894
More than Twelve Months	841,760	691,297
Total	1,682,248	1,181,191
Unrealized Losses
Less than Twelve Months	(23,885)	(2,932)
More than Twelve Months	(45,324)	(19,226)
Total	(69,209)	(22,158)
U.S. Treasury securities
Fair Value
Less than Twelve Months	897	916
More than Twelve Months	47,467	48,726
Total	48,364	49,642
Unrealized Losses
Less than Twelve Months	(16)	(2)
More than Twelve Months	(2,601)	(1,353)
Total	(2,617)	(1,355)
U.S. government agency securities
Fair Value
Less than Twelve Months	5,502	31,177
More than Twelve Months	23,561	0
Total	29,063	31,177
Unrealized Losses
Less than Twelve Months	(77)	(37)
More than Twelve Months	(1,095)	0
Total	(1,172)	(37)
Obligations of states and state agencies
Fair Value
Less than Twelve Months	86,962	13,337
More than Twelve Months	12,297	7,792
Total	99,259	21,129
Unrealized Losses
Less than Twelve Months	(2,850)	(131)
More than Twelve Months	(494)	(243)
Total	(3,344)	(374)
Municipal bonds
Fair Value
Less than Twelve Months	58,259	31,669
More than Twelve Months	22,850	12,133
Total	81,109	43,802
Unrealized Losses
Less than Twelve Months	(1,205)	(256)
More than Twelve Months	(904)	(308)
Total	(2,109)	(564)
Obligations of states and political subdivisions
Fair Value
Less than Twelve Months	145,221	45,006
More than Twelve Months	35,147	19,925
Total	180,368	64,931
Unrealized Losses
Less than Twelve Months	(4,055)	(387)
More than Twelve Months	(1,398)	(551)
Total	(5,453)	(938)
Residential mortgage-backed securities
Fair Value
Less than Twelve Months	667,551	406,940
More than Twelve Months	723,522	599,167
Total	1,391,073	1,006,107
Unrealized Losses
Less than Twelve Months	(19,418)	(2,461)
More than Twelve Months	(39,602)	(16,201)
Total	(59,020)	(18,662)
Trust preferred securities
Fair Value
Less than Twelve Months	0	0
More than Twelve Months	1,590	3,214
Total	1,590	3,214
Unrealized Losses
Less than Twelve Months	0	0
More than Twelve Months	(199)	(512)
Total	(199)	(512)
Corporate and other debt securities
Fair Value
Less than Twelve Months	21,317	5,855
More than Twelve Months	10,473	15,115
Total	31,790	20,970
Unrealized Losses
Less than Twelve Months	(319)	(45)
More than Twelve Months	(429)	(115)
Total	$ (748)	(160)
Equity securities
Fair Value
Less than Twelve Months		0
More than Twelve Months		5,150
Total		5,150
Unrealized Losses
Less than Twelve Months		0
More than Twelve Months		(494)
Total		$ (494)