Fair Value - Assets and Liabilities Recorded at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets
Investment securities	$ 5,314,973	$ 5,077,514
Fair Value Measurements, Recurring
Assets
Investment securities	5,314,973	5,077,514
Other assets	19,831	15,982
Liabilities
Other liabilities	(24,833)	(31,202)
Total
Net Assets (Liabilities)	5,309,971	5,062,294
Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	5,314,973	5,077,514
Other assets	19,831	15,982
Liabilities
Other liabilities	(18,858)	(23,742)
Total
Net Assets (Liabilities)	5,315,946	5,069,754
Fair Value Measurements, Recurring | Level 3
Liabilities
Other liabilities	(5,975)	(7,460)
Total
Net Assets (Liabilities)	(5,975)	(7,460)
U.S. Treasury securities
Assets
Investment securities	396,766	392,473
U.S. Treasury securities | Fair Value Measurements, Recurring
Assets
Investment securities	396,766	392,473
U.S. Treasury securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	396,766	392,473
Government-sponsored enterprises debt securities
Assets
Investment securities	245,138	242,667
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring
Assets
Investment securities	245,138	242,667
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	245,138	242,667
Government agency mortgage-backed securities
Assets
Investment securities	159,787	185,663
Government agency mortgage-backed securities | Fair Value Measurements, Recurring
Assets
Investment securities	159,787	185,663
Government agency mortgage-backed securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	159,787	185,663
Government-sponsored enterprises mortgage-backed securities
Assets
Investment securities	184,139	204,385
Government-sponsored enterprises mortgage-backed securities | Fair Value Measurements, Recurring
Assets
Investment securities	184,139	204,385
Government-sponsored enterprises mortgage-backed securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	184,139	204,385
Non-government asset-backed securities
Assets
Investment securities	101	12,583
Non-government asset-backed securities | Fair Value Measurements, Recurring
Assets
Investment securities	101	12,583
Non-government asset-backed securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	101	12,583
Collateralized mortgage obligations: Government agency
Assets
Investment securities	3,513,904	3,351,822
Collateralized mortgage obligations: Government agency | Fair Value Measurements, Recurring
Assets
Investment securities	3,513,904	3,351,822
Collateralized mortgage obligations: Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	3,513,904	3,351,822
Collateralized mortgage obligations: Government-sponsored enterprises
Assets
Investment securities	815,138	687,921
Collateralized mortgage obligations: Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Investment securities	815,138	687,921
Collateralized mortgage obligations: Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	$ 815,138	$ 687,921