Fair Value - Assets and Liabilities Recorded at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Assets
Investment securities	$ 5,260,262	$ 5,077,514
Fair Value Measurements, Recurring
Assets
Investment securities	5,260,262	5,077,514
Other assets	17,707	15,982
Liabilities
Other liabilities	(31,180)	(31,202)
Total
Net Assets (Liabilities)	5,246,789	5,062,294
Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	5,260,262	5,077,514
Other assets	17,707	15,982
Liabilities
Other liabilities	(24,193)	(23,742)
Total
Net Assets (Liabilities)	5,253,776	5,069,754
Fair Value Measurements, Recurring | Level 3
Liabilities
Other liabilities	(6,987)	(7,460)
Total
Net Assets (Liabilities)	(6,987)	(7,460)
U.S. Treasury securities
Assets
Investment securities	393,811	392,473
U.S. Treasury securities | Fair Value Measurements, Recurring
Assets
Investment securities	393,811	392,473
U.S. Treasury securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	393,811	392,473
Government-sponsored enterprises debt securities
Assets
Investment securities	243,267	242,667
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring
Assets
Investment securities	243,267	242,667
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	243,267	242,667
Government agency mortgage-backed securities
Assets
Investment securities	175,536	185,663
Government agency mortgage-backed securities | Fair Value Measurements, Recurring
Assets
Investment securities	175,536	185,663
Government agency mortgage-backed securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	175,536	185,663
Government-sponsored enterprises mortgage-backed securities
Assets
Investment securities	196,315	204,385
Government-sponsored enterprises mortgage-backed securities | Fair Value Measurements, Recurring
Assets
Investment securities	196,315	204,385
Government-sponsored enterprises mortgage-backed securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	196,315	204,385
Non-government asset-backed securities
Assets
Investment securities	8,034	12,583
Non-government asset-backed securities | Fair Value Measurements, Recurring
Assets
Investment securities	8,034	12,583
Non-government asset-backed securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	8,034	12,583
Collateralized mortgage obligations: Government agency
Assets
Investment securities	3,393,256	3,351,822
Collateralized mortgage obligations: Government agency | Fair Value Measurements, Recurring
Assets
Investment securities	3,393,256	3,351,822
Collateralized mortgage obligations: Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	3,393,256	3,351,822
Collateralized mortgage obligations: Government-sponsored enterprises
Assets
Investment securities	850,043	687,921
Collateralized mortgage obligations: Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Investment securities	850,043	687,921
Collateralized mortgage obligations: Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	$ 850,043	$ 687,921