Fair Value - Assets and Liabilities Recorded at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Assets
Investment securities	$ 4,157,082	$ 4,498,342
Fair Value Measurements, Recurring
Assets
Investment securities	4,157,082	4,498,342
Other assets	85,883	12,336
Liabilities
Other liabilities	(1,542)	(14,692)
Total
Net Assets (Liabilities)	4,241,423	4,495,986
Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	4,157,082	4,498,342
Other assets	85,883	12,336
Liabilities
Other liabilities	(958)	(12,085)
Total
Net Assets (Liabilities)	4,242,007	4,498,593
Fair Value Measurements, Recurring | Level 3
Liabilities
Other liabilities	(584)	(2,607)
Total
Net Assets (Liabilities)	(584)	(2,607)
U.S. Treasury securities
Assets
Investment securities	29,738	389,470
U.S. Treasury securities | Fair Value Measurements, Recurring
Assets
Investment securities	29,738	389,470
U.S. Treasury securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	29,738	389,470
Government agency debt securities
Assets
Investment securities	24,472
Government agency debt securities | Fair Value Measurements, Recurring
Assets
Investment securities	24,472
Government agency debt securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	24,472
Government-sponsored enterprises debt securities
Assets
Investment securities	126,649	241,594
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring
Assets
Investment securities	126,649	241,594
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	126,649	241,594
Government agency mortgage-backed securities
Assets
Investment securities	343,905	411,536
Government agency mortgage-backed securities | Fair Value Measurements, Recurring
Assets
Investment securities	343,905	411,536
Government agency mortgage-backed securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	343,905	411,536
Government-sponsored enterprises mortgage-backed securities
Assets
Investment securities	371,949	150,847
Government-sponsored enterprises mortgage-backed securities | Fair Value Measurements, Recurring
Assets
Investment securities	371,949	150,847
Government-sponsored enterprises mortgage-backed securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	371,949	150,847
Collateralized mortgage obligations: Government agency
Assets
Investment securities	2,486,524	2,682,449
Collateralized mortgage obligations: Government agency | Fair Value Measurements, Recurring
Assets
Investment securities	2,486,524	2,682,449
Collateralized mortgage obligations: Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	2,486,524	2,682,449
Collateralized mortgage obligations: Government-sponsored enterprises
Assets
Investment securities	773,845	602,592
Collateralized mortgage obligations: Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Investment securities	773,845	602,592
Collateralized mortgage obligations: Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	$ 773,845	602,592
Debt securities issued by states and political subdivisions
Assets
Investment securities		19,854
Debt securities issued by states and political subdivisions | Fair Value Measurements, Recurring
Assets
Investment securities		19,854
Debt securities issued by states and political subdivisions | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities		$ 19,854