Investment Securities - Unrealized Losses and Fair Value of Held to Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	$ 655,542	$ 365,182
Less than Twelve Months, Unrealized Losses	(16,398)	(2,702)
More than Twelve Months, Fair Value	637,421	663,238
More than Twelve Months, Unrealized Losses	(39,306)	(27,788)
Total, Fair Value	1,292,963	1,028,420
Total, Unrealized Losses	(55,704)	(30,490)
U.S. government agency securities
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	6,527
Less than Twelve Months, Unrealized Losses	(188)
More than Twelve Months, Fair Value	0
More than Twelve Months, Unrealized Losses	0
Total, Fair Value	6,527
Total, Unrealized Losses	(188)
Obligations of states and state agencies
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	125,371	6,342
Less than Twelve Months, Unrealized Losses	(2,503)	(50)
More than Twelve Months, Fair Value	46,899	53,034
More than Twelve Months, Unrealized Losses	(3,359)	(1,603)
Total, Fair Value	172,270	59,376
Total, Unrealized Losses	(5,862)	(1,653)
Municipal bonds
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	87,785	4,644
Less than Twelve Months, Unrealized Losses	(1,546)	(25)
More than Twelve Months, Fair Value	533	561
More than Twelve Months, Unrealized Losses	(31)	(3)
Total, Fair Value	88,318	5,205
Total, Unrealized Losses	(1,577)	(28)
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	213,156	10,986
Less than Twelve Months, Unrealized Losses	(4,049)	(75)
More than Twelve Months, Fair Value	47,432	53,595
More than Twelve Months, Unrealized Losses	(3,390)	(1,606)
Total, Fair Value	260,588	64,581
Total, Unrealized Losses	(7,439)	(1,681)
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	425,381	344,216
Less than Twelve Months, Unrealized Losses	(12,139)	(2,357)
More than Twelve Months, Fair Value	555,355	570,969
More than Twelve Months, Unrealized Losses	(29,069)	(16,386)
Total, Fair Value	980,736	915,185
Total, Unrealized Losses	(41,208)	(18,743)
Trust preferred securities
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	0	0
Less than Twelve Months, Unrealized Losses	0	0
More than Twelve Months, Fair Value	29,903	38,674
More than Twelve Months, Unrealized Losses	(6,577)	(9,796)
Total, Fair Value	29,903	38,674
Total, Unrealized Losses	(6,577)	(9,796)
Corporate and other debt securities
Schedule of Held-to-maturity Securities
Less than Twelve Months, Fair Value	10,478	9,980
Less than Twelve Months, Unrealized Losses	(22)	(270)
More than Twelve Months, Fair Value	4,731	0
More than Twelve Months, Unrealized Losses	(270)	0
Total, Fair Value	15,209	9,980
Total, Unrealized Losses	$ (292)	$ (270)