Fair Value - Assets and Liabilities Recorded at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets
Investment securities	$ 4,485,660	$ 4,498,342
Fair Value Measurements, Recurring
Assets
Investment securities	4,485,660	4,498,342
Other assets	29,037	12,336
Liabilities
Other liabilities	(4,090)	(14,692)
Total
Net Assets (Liabilities)	4,510,607	4,495,986
Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	4,485,660	4,498,342
Other assets	29,037	12,336
Liabilities
Other liabilities	(2,251)	(12,085)
Total
Net Assets (Liabilities)	4,512,446	4,498,593
Fair Value Measurements, Recurring | Level 3
Liabilities
Other liabilities	(1,839)	(2,607)
Total
Net Assets (Liabilities)	(1,839)	(2,607)
Government agency debt securities
Assets
Investment securities	24,765
Government agency debt securities | Fair Value Measurements, Recurring
Assets
Investment securities	24,765
Government agency debt securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	24,765
U.S. Treasury securities
Assets
Investment securities		389,470
U.S. Treasury securities | Fair Value Measurements, Recurring
Assets
Investment securities		389,470
U.S. Treasury securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities		389,470
Government-sponsored enterprises debt securities
Assets
Investment securities	161,595	241,594
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring
Assets
Investment securities	161,595	241,594
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	161,595	241,594
Government agency mortgage-backed securities
Assets
Investment securities	371,035	411,536
Government agency mortgage-backed securities | Fair Value Measurements, Recurring
Assets
Investment securities	371,035	411,536
Government agency mortgage-backed securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	371,035	411,536
Government-sponsored enterprises mortgage-backed securities
Assets
Investment securities	127,896	150,847
Government-sponsored enterprises mortgage-backed securities | Fair Value Measurements, Recurring
Assets
Investment securities	127,896	150,847
Government-sponsored enterprises mortgage-backed securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	127,896	150,847
Collateralized mortgage obligations: Government agency
Assets
Investment securities	2,730,166	2,682,449
Collateralized mortgage obligations: Government agency | Fair Value Measurements, Recurring
Assets
Investment securities	2,730,166	2,682,449
Collateralized mortgage obligations: Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	2,730,166	2,682,449
Collateralized mortgage obligations: Government-sponsored enterprises
Assets
Investment securities	1,070,203	602,592
Collateralized mortgage obligations: Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Investment securities	1,070,203	602,592
Collateralized mortgage obligations: Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	$ 1,070,203	602,592
States and political subdivisions
Assets
Investment securities		19,854
States and political subdivisions | Fair Value Measurements, Recurring
Assets
Investment securities		19,854
States and political subdivisions | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities		$ 19,854