Investment Securities - Unrealized Losses and Fair Value of Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value
Less than Twelve Months	$ 1,325,414	$ 37,894
More than Twelve Months	38,568	31,100
Total	1,363,982	68,994
Unrealized Losses
Less than Twelve Months	(18,347)	(351)
More than Twelve Months	(5,609)	(7,374)
Total	(23,956)	(7,725)
Obligations of states and state agencies
Fair Value
Less than Twelve Months	10,541	5,546
More than Twelve Months	5,519	0
Total	16,060	5,546
Unrealized Losses
Less than Twelve Months	(218)	(105)
More than Twelve Months	(75)	0
Total	(293)	(105)
Municipal bonds
Fair Value
Less than Twelve Months	14,917
More than Twelve Months	0
Total	14,917
Unrealized Losses
Less than Twelve Months	(142)
More than Twelve Months	0
Total	(142)	0
Obligations of states and political subdivisions
Fair Value
Less than Twelve Months	25,458
More than Twelve Months	5,519
Total	30,977
Unrealized Losses
Less than Twelve Months	(360)
More than Twelve Months	(75)
Total	(435)	(105)
Residential mortgage-backed securities
Fair Value
Less than Twelve Months	1,292,965	21,599
More than Twelve Months	2,546	2,470
Total	1,295,511	24,069
Unrealized Losses
Less than Twelve Months	(17,978)	(245)
More than Twelve Months	(23)	(9)
Total	(18,001)	(254)
Trust preferred securities
Fair Value
Less than Twelve Months	0	0
More than Twelve Months	30,503	28,630
Total	30,503	28,630
Unrealized Losses
Less than Twelve Months	0	0
More than Twelve Months	(5,511)	(7,365)
Total	(5,511)	(7,365)
Corporate and other debt securities
Fair Value
Less than Twelve Months	6,991	10,749
More than Twelve Months	0	0
Total	6,991	10,749
Unrealized Losses
Less than Twelve Months	(9)	(1)
More than Twelve Months	0	0
Total	$ (9)	$ (1)