Securities (Summary of Gross Unrealized Losses and Fair Value of the Investments with Unrealized Losses) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months Fair Value	$ 2,306,179	$ 880,507
Less than 12 Months Unrealized Losses	(58,072)	(6,453)
12 Months or More Fair Value	16,532	18,597
12 Months or More Unrealized Losses	(719)	(815)
Total Fair Value	2,322,711	899,104
Total Unrealized Losses	(58,791)	(7,268)
U.S. Government Agency and Government-Sponsored Enterprise Mortgage-Backed Securities and Collateralized Mortgage Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months Fair Value	1,661,107	575,329
Less than 12 Months Unrealized Losses	(44,484)	(3,728)
12 Months or More Fair Value	16,463	18,527
12 Months or More Unrealized Losses	(719)	(815)
Total Fair Value	1,677,570	593,856
Total Unrealized Losses	(45,203)	(4,543)
Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months Fair Value	183,636	143,764
Less than 12 Months Unrealized Losses	(5,656)	(2,076)
12 Months or More Fair Value	69	70
12 Months or More Unrealized Losses	0	0
Total Fair Value	183,705	143,834
Total Unrealized Losses	(5,656)	(2,076)
State and Municipal Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months Fair Value	311,953	86,471
Less than 12 Months Unrealized Losses	(6,365)	(592)
12 Months or More Fair Value	0	0
12 Months or More Unrealized Losses	0	0
Total Fair Value	311,953	86,471
Total Unrealized Losses	(6,365)	(592)
U.S. Government and Government-Sponsored Enterprise Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 Months Fair Value	149,483	74,943
Less than 12 Months Unrealized Losses	(1,567)	(57)
12 Months or More Fair Value	0	0
12 Months or More Unrealized Losses	0	0
Total Fair Value	149,483	74,943
Total Unrealized Losses	$ (1,567)	$ (57)