Fair Value - Assets and Liabilities Recorded at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Assets
Fair value	$ 6,692,479	$ 6,071,415
Fair Value Measurements, Recurring
Assets
Fair value	6,692,479	6,071,415
Other assets	92,912	141,586
Liabilities
Other liabilities	(4,522)	(5,837)
Total
Net Assets (Liabilities)	6,780,869	6,207,164
Fair Value Measurements, Recurring | Level 1
Assets
Other assets	12,730	11,691
Total
Net Assets (Liabilities)	12,730	11,691
Fair Value Measurements, Recurring | Level 2
Assets
Fair value	6,692,479	6,071,415
Other assets	80,182	129,895
Liabilities
Other liabilities	(1,140)	(1,283)
Total
Net Assets (Liabilities)	6,771,521	6,200,027
Fair Value Measurements, Recurring | Level 3
Liabilities
Other liabilities	(3,382)	(4,554)
Total
Net Assets (Liabilities)	(3,382)	(4,554)
U.S. Treasury and government agency debt securities
Assets
Fair value	175,724	171,421
U.S. Treasury and government agency debt securities | Fair Value Measurements, Recurring
Assets
Fair value	175,724	171,421
U.S. Treasury and government agency debt securities | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	175,724	171,421
Residential - Government agency
Assets
Fair value	133,894	160,462
Residential - Government agency | Fair Value Measurements, Recurring
Assets
Fair value	133,894	160,462
Residential - Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	133,894	160,462
Residential - Government-sponsored enterprises
Assets
Fair value	800,920	447,200
Residential - Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Fair value	800,920	447,200
Residential - Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	800,920	447,200
Commercial - Government agency
Assets
Fair value	527,516	599,650
Commercial - Government agency | Fair Value Measurements, Recurring
Assets
Fair value	527,516	599,650
Commercial - Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	527,516	599,650
Commercial - Government-sponsored enterprises
Assets
Fair value	1,134,175	932,157
Commercial - Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Fair value	1,134,175	932,157
Commercial - Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	1,134,175	932,157
Government agency
Assets
Fair value	1,845,098	1,933,553
Government agency | Fair Value Measurements, Recurring
Assets
Fair value	1,845,098	1,933,553
Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	1,845,098	1,933,553
Government-sponsored enterprises
Assets
Fair value	2,075,152	1,826,972
Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Fair value	2,075,152	1,826,972
Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	$ 2,075,152	$ 1,826,972