Fair Value - Assets and Liabilities Recorded at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Assets
Fair value	$ 6,953,930	$ 6,071,415
Fair Value Measurements, Recurring
Assets
Fair value	6,953,930	6,071,415
Other assets	95,613	141,586
Liabilities
Other liabilities	(3,104)	(5,837)
Total
Net Assets (Liabilities)	7,046,439	6,207,164
Fair Value Measurements, Recurring | Level 1
Assets
Other assets	12,901	11,691
Total
Net Assets (Liabilities)	12,901	11,691
Fair Value Measurements, Recurring | Level 2
Assets
Fair value	6,953,930	6,071,415
Other assets	82,712	129,895
Liabilities
Other liabilities	(972)	(1,283)
Total
Net Assets (Liabilities)	7,035,670	6,200,027
Fair Value Measurements, Recurring | Level 3
Liabilities
Other liabilities	(2,132)	(4,554)
Total
Net Assets (Liabilities)	(2,132)	(4,554)
U.S. Treasury and government agency debt securities
Assets
Fair value	180,772	171,421
U.S. Treasury and government agency debt securities | Fair Value Measurements, Recurring
Assets
Fair value	180,772	171,421
U.S. Treasury and government agency debt securities | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	180,772	171,421
Residential - Government agency
Assets
Fair value	113,289	160,462
Residential - Government agency | Fair Value Measurements, Recurring
Assets
Fair value	113,289	160,462
Residential - Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	113,289	160,462
Residential - Government-sponsored enterprises
Assets
Fair value	1,263,448	447,200
Residential - Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Fair value	1,263,448	447,200
Residential - Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	1,263,448	447,200
Commercial - Government agency
Assets
Fair value	491,447	599,650
Commercial - Government agency | Fair Value Measurements, Recurring
Assets
Fair value	491,447	599,650
Commercial - Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	491,447	599,650
Commercial - Government-sponsored enterprises
Assets
Fair value	1,223,805	932,157
Commercial - Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Fair value	1,223,805	932,157
Commercial - Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	1,223,805	932,157
Government agency
Assets
Fair value	1,662,134	1,933,553
Government agency | Fair Value Measurements, Recurring
Assets
Fair value	1,662,134	1,933,553
Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	1,662,134	1,933,553
Government-sponsored enterprises
Assets
Fair value	2,019,035	1,826,972
Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Fair value	2,019,035	1,826,972
Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	$ 2,019,035	$ 1,826,972