Investment Securities - Unrealized Gross Losses and Fair Values of Securities in a Continuous Loss Position (Details)	Jun. 30, 2021 USD ($) security	Dec. 31, 2020 USD ($) security
Securities in the available-for-sale portfolio in a continuous loss position
Number of individual securities in a continuous loss position | security	162	50
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	$ (59,399,000)	$ (10,705,000)
12 Months or More Unrealized Losses	(217,000)	(25,000)
Total Unrealized Losses	(59,616,000)	(10,730,000)
Time in Continuous Loss, Fair Value
Less Than 12 Months, Fair Value	3,499,015,000	1,068,883,000
12 Months or More Fair Value	7,064,000	6,190,000
Total Fair Value	3,506,079,000	1,075,073,000
Credit losses	0	0
Amortized cost basis	0	0
U.S. Treasury and government agency debt securities
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(2,127,000)	(61,000)
Total Unrealized Losses	(2,127,000)	(61,000)
Time in Continuous Loss, Fair Value
Less Than 12 Months, Fair Value	120,987,000	38,507,000
Total Fair Value	120,987,000	38,507,000
Residential - Government-sponsored enterprises
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(4,360,000)	(725,000)
Total Unrealized Losses	(4,360,000)	(725,000)
Time in Continuous Loss, Fair Value
Less Than 12 Months, Fair Value	618,414,000	64,987,000
Total Fair Value	618,414,000	64,987,000
Commercial - Government agency
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(2,899,000)	(119,000)
12 Months or More Unrealized Losses	(217,000)
Total Unrealized Losses	(3,116,000)	(119,000)
Time in Continuous Loss, Fair Value
Less Than 12 Months, Fair Value	193,041,000	32,346,000
12 Months or More Fair Value	7,064,000
Total Fair Value	200,105,000	32,346,000
Commercial - Government-sponsored enterprises
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(27,342,000)	(7,983,000)
Total Unrealized Losses	(27,342,000)	(7,983,000)
Time in Continuous Loss, Fair Value
Less Than 12 Months, Fair Value	865,899,000	427,759,000
Total Fair Value	865,899,000	427,759,000
Government agency
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(5,667,000)	(994,000)
12 Months or More Unrealized Losses		(25,000)
Total Unrealized Losses	(5,667,000)	(1,019,000)
Time in Continuous Loss, Fair Value
Less Than 12 Months, Fair Value	463,392,000	209,124,000
12 Months or More Fair Value		6,190,000
Total Fair Value	463,392,000	215,314,000
Government-sponsored enterprises
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(17,004,000)	(823,000)
Total Unrealized Losses	(17,004,000)	(823,000)
Time in Continuous Loss, Fair Value
Less Than 12 Months, Fair Value	1,237,282,000	296,160,000
Total Fair Value	$ 1,237,282,000	$ 296,160,000