Fair Value - Assets and Liabilities Recorded at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets
Fair value	$ 6,071,415	$ 4,075,644
Fair Value Measurements, Recurring
Assets
Fair value	6,071,415	4,075,644
Other assets	141,586	63,539
Liabilities
Other liabilities	(5,837)	(4,915)
Total
Net Assets (Liabilities)	6,207,164	4,134,268
Fair Value Measurements, Recurring | Level 1
Assets
Other assets	11,691
Total
Net Assets (Liabilities)	11,691
Fair Value Measurements, Recurring | Level 2
Assets
Fair value	6,071,415	4,075,644
Other assets	129,895	63,539
Liabilities
Other liabilities	(1,283)	(682)
Total
Net Assets (Liabilities)	6,200,027	4,138,501
Fair Value Measurements, Recurring | Level 3
Liabilities
Other liabilities	(4,554)	(4,233)
Total
Net Assets (Liabilities)	(4,554)	(4,233)
U.S. Treasury securities
Assets
Fair value	171,421	29,888
U.S. Treasury securities | Fair Value Measurements, Recurring
Assets
Fair value	171,421	29,888
U.S. Treasury securities | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	171,421	29,888
Government-sponsored enterprises debt securities
Assets
Fair value		101,439
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring
Assets
Fair value		101,439
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring | Level 2
Assets
Fair value		101,439
Residential - Government agency
Assets
Fair value	160,462	291,209
Residential - Government agency | Fair Value Measurements, Recurring
Assets
Fair value	160,462	291,209
Residential - Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	160,462	291,209
Residential - Government-sponsored enterprises
Assets
Fair value	447,200	399,492
Residential - Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Fair value	447,200	399,492
Residential - Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	447,200	399,492
Commercial - Government agency
Assets
Fair value	599,650
Commercial - Government agency | Fair Value Measurements, Recurring
Assets
Fair value	599,650
Commercial - Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	599,650
Commercial - Government-sponsored enterprises
Assets
Fair value	932,157	101,719
Commercial - Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Fair value	932,157	101,719
Commercial - Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	932,157	101,719
Government agency
Assets
Fair value	1,933,553	2,381,278
Government agency | Fair Value Measurements, Recurring
Assets
Fair value	1,933,553	2,381,278
Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	1,933,553	2,381,278
Government-sponsored enterprises
Assets
Fair value	1,826,972	770,619
Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Fair value	1,826,972	770,619
Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	$ 1,826,972	$ 770,619