Investment Securities - Unrealized Losses and Fair Value of Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value
Less than Twelve Months	$ 908,811	$ 37,894
More than Twelve Months	36,681	31,100
Total	945,492	68,994
Unrealized Losses
Less than Twelve Months	(18,817)	(351)
More than Twelve Months	(6,795)	(7,374)
Total	(25,612)	(7,725)
Obligations of states and state agencies
Fair Value
Less than Twelve Months	809	5,546
More than Twelve Months	5,521	0
Total	6,330	5,546
Unrealized Losses
Less than Twelve Months	(30)	(105)
More than Twelve Months	(111)	0
Total	(141)	(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	(2)
More than Twelve Months	0
Total	(2)	0
Obligations of states and political subdivisions
Fair Value
Less than Twelve Months	2,137
More than Twelve Months	5,521
Total	7,658
Unrealized Losses
Less than Twelve Months	(32)
More than Twelve Months	(111)
Total	(143)	(105)
Residential mortgage-backed securities
Fair Value
Less than Twelve Months	903,746	21,599
More than Twelve Months	1,838	2,470
Total	905,584	24,069
Unrealized Losses
Less than Twelve Months	(18,713)	(245)
More than Twelve Months	(6)	(9)
Total	(18,719)	(254)
Trust preferred securities
Fair Value
Less than Twelve Months	0	0
More than Twelve Months	29,322	28,630
Total	29,322	28,630
Unrealized Losses
Less than Twelve Months	0	0
More than Twelve Months	(6,678)	(7,365)
Total	(6,678)	(7,365)
Corporate and other debt securities
Fair Value
Less than Twelve Months	2,928	10,749
More than Twelve Months	0	0
Total	2,928	10,749
Unrealized Losses
Less than Twelve Months	(72)	(1)
More than Twelve Months	0	0
Total	$ (72)	$ (1)