Fair Value - Assets and Liabilities Recorded at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Assets
Fair value	$ 7,953,727	$ 6,071,415
Fair Value Measurements, Recurring
Assets
Fair value	7,953,727	6,071,415
Other assets	75,377	141,586
Liabilities
Other liabilities	(1,708)	(5,837)
Total
Net Assets (Liabilities)	8,027,396	6,207,164
Fair Value Measurements, Recurring | Level 1
Assets
Other assets	7,327	11,691
Total
Net Assets (Liabilities)	7,327	11,691
Fair Value Measurements, Recurring | Level 2
Assets
Fair value	7,953,727	6,071,415
Other assets	68,050	129,895
Liabilities
Other liabilities	(856)	(1,283)
Total
Net Assets (Liabilities)	8,020,921	6,200,027
Fair Value Measurements, Recurring | Level 3
Liabilities
Other liabilities	(852)	(4,554)
Total
Net Assets (Liabilities)	(852)	(4,554)
U.S. Treasury and government agency debt securities
Assets
Fair value	187,861	171,421
U.S. Treasury and government agency debt securities | Fair Value Measurements, Recurring
Assets
Fair value	187,861	171,421
U.S. Treasury and government agency debt securities | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	187,861	171,421
Residential - Government agency
Assets
Fair value	98,134	160,462
Residential - Government agency | Fair Value Measurements, Recurring
Assets
Fair value	98,134	160,462
Residential - Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	98,134	160,462
Residential - Government-sponsored enterprises
Assets
Fair value	1,458,878	447,200
Residential - Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Fair value	1,458,878	447,200
Residential - Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	1,458,878	447,200
Commercial - Government agency
Assets
Fair value	448,594	599,650
Commercial - Government agency | Fair Value Measurements, Recurring
Assets
Fair value	448,594	599,650
Commercial - Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	448,594	599,650
Commercial - Government-sponsored enterprises
Assets
Fair value	1,301,249	932,157
Commercial - Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Fair value	1,301,249	932,157
Commercial - Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	1,301,249	932,157
Government agency
Assets
Fair value	2,059,864	1,933,553
Government agency | Fair Value Measurements, Recurring
Assets
Fair value	2,059,864	1,933,553
Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	2,059,864	1,933,553
Government-sponsored enterprises
Assets
Fair value	2,399,147	1,826,972
Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Fair value	2,399,147	1,826,972
Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	$ 2,399,147	$ 1,826,972