Investment Securities - Unrealized Losses and Fair Value Available for Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value
Less than Twelve Months	$ 1,095,370	$ 489,894
More than Twelve Months	638,437	691,297
Total	1,733,807	1,181,191
Unrealized Losses
Less than Twelve Months	(26,808)	(2,932)
More than Twelve Months	(30,594)	(19,226)
Total	(57,402)	(22,158)
U.S. Treasury securities
Fair Value
Less than Twelve Months	901	916
More than Twelve Months	47,766	48,726
Total	48,667	49,642
Unrealized Losses
Less than Twelve Months	(13)	(2)
More than Twelve Months	(2,306)	(1,353)
Total	(2,319)	(1,355)
U.S. government agency securities
Fair Value
Less than Twelve Months	30,485	31,177
More than Twelve Months	0	0
Total	30,485	31,177
Unrealized Losses
Less than Twelve Months	(1,240)	(37)
More than Twelve Months	0	0
Total	(1,240)	(37)
Obligations of states and state agencies
Fair Value
Less than Twelve Months	96,617	13,337
More than Twelve Months	8,041	7,792
Total	104,658	21,129
Unrealized Losses
Less than Twelve Months	(2,301)	(131)
More than Twelve Months	(335)	(243)
Total	(2,636)	(374)
Municipal bonds
Fair Value
Less than Twelve Months	69,301	31,669
More than Twelve Months	12,010	12,133
Total	81,311	43,802
Unrealized Losses
Less than Twelve Months	(1,251)	(256)
More than Twelve Months	(419)	(308)
Total	(1,670)	(564)
Obligations of states and political subdivisions
Fair Value
Less than Twelve Months	165,918	45,006
More than Twelve Months	20,051	19,925
Total	185,969	64,931
Unrealized Losses
Less than Twelve Months	(3,552)	(387)
More than Twelve Months	(754)	(551)
Total	(4,306)	(938)
Residential mortgage-backed securities
Fair Value
Less than Twelve Months	875,444	406,940
More than Twelve Months	553,585	599,167
Total	1,429,029	1,006,107
Unrealized Losses
Less than Twelve Months	(21,543)	(2,461)
More than Twelve Months	(26,982)	(16,201)
Total	(48,525)	(18,662)
Trust preferred securities
Fair Value
Less than Twelve Months	0	0
More than Twelve Months	2,279	3,214
Total	2,279	3,214
Unrealized Losses
Less than Twelve Months	0	0
More than Twelve Months	(304)	(512)
Total	(304)	(512)
Corporate and other debt securities
Fair Value
Less than Twelve Months	22,622	5,855
More than Twelve Months	14,756	15,115
Total	37,378	20,970
Unrealized Losses
Less than Twelve Months	(460)	(45)
More than Twelve Months	(248)	(115)
Total	$ (708)	(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)