Investment Securities - Unrealized Losses and Fair Value of Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Fair Value
Less than Twelve Months	$ 992,724	$ 37,894
More than Twelve Months	40,199	31,100
Total	1,032,923	68,994
Unrealized Losses
Less than Twelve Months	(11,634)	(351)
More than Twelve Months	(6,091)	(7,374)
Total	(17,725)	(7,725)
Obligations of states and state agencies
Fair Value
Less than Twelve Months	0	5,546
More than Twelve Months	5,570	0
Total	5,570	5,546
Unrealized Losses
Less than Twelve Months	0	(105)
More than Twelve Months	(43)	0
Total	(43)	(105)
Municipal bonds
Fair Value
Less than Twelve Months	1,328
More than Twelve Months	0
Total	1,328
Unrealized Losses
Less than Twelve Months	(18)
More than Twelve Months	0
Total	(18)	0
Obligations of states and political subdivisions
Fair Value
Less than Twelve Months	1,328
More than Twelve Months	5,570
Total	6,898
Unrealized Losses
Less than Twelve Months	(18)
More than Twelve Months	(43)
Total	(61)	(105)
Residential mortgage-backed securities
Fair Value
Less than Twelve Months	983,406	21,599
More than Twelve Months	4,608	2,470
Total	988,014	24,069
Unrealized Losses
Less than Twelve Months	(11,606)	(245)
More than Twelve Months	(62)	(9)
Total	(11,668)	(254)
Trust preferred securities
Fair Value
Less than Twelve Months	0	0
More than Twelve Months	30,021	28,630
Total	30,021	28,630
Unrealized Losses
Less than Twelve Months	0	0
More than Twelve Months	(5,986)	(7,365)
Total	(5,986)	(7,365)
Corporate and other debt securities
Fair Value
Less than Twelve Months	7,990	10,749
More than Twelve Months	0	0
Total	7,990	10,749
Unrealized Losses
Less than Twelve Months	(10)	(1)
More than Twelve Months	0	0
Total	$ (10)	$ (1)