Fair Value - Assets and Liabilities Recorded at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Assets
Investment securities	$ 5,077,514	$ 4,027,265
Fair Value, Measurements, Recurring
Assets
Investment securities	5,077,514	4,027,265
Other assets	15,982	11,002
Liabilities
Other liabilities	(31,202)	(23,122)
Total
Net Assets (Liabilities)	5,062,294	4,015,145
Fair Value, Measurements, Recurring | Level 2
Assets
Investment securities	5,077,514	4,027,265
Other assets	15,982	11,002
Liabilities
Other liabilities	(23,742)	(23,122)
Total
Net Assets (Liabilities)	5,069,754	4,015,145
Fair Value, Measurements, Recurring | Level 3
Liabilities
Other liabilities	(7,460)
Total
Net Assets (Liabilities)	(7,460)
U.S. Treasury securities
Assets
Investment securities	392,473	499,976
U.S. Treasury securities | Fair Value, Measurements, Recurring
Assets
Investment securities	392,473	499,976
U.S. Treasury securities | Fair Value, Measurements, Recurring | Level 2
Assets
Investment securities	392,473	499,976
Government-sponsored enterprises debt securities
Assets
Investment securities	242,667	95,824
Government-sponsored enterprises debt securities | Fair Value, Measurements, Recurring
Assets
Investment securities	242,667	95,824
Government-sponsored enterprises debt securities | Fair Value, Measurements, Recurring | Level 2
Assets
Investment securities	242,667	95,824
Government agency mortgage-backed securities
Assets
Investment securities	185,663	55,982
Government agency mortgage-backed securities | Fair Value, Measurements, Recurring
Assets
Investment securities	185,663	55,982
Government agency mortgage-backed securities | Fair Value, Measurements, Recurring | Level 2
Assets
Investment securities	185,663	55,982
Government-sponsored enterprises mortgage-backed securities
Assets
Investment securities	204,385	10,745
Government-sponsored enterprises mortgage-backed securities | Fair Value, Measurements, Recurring
Assets
Investment securities	204,385	10,745
Government-sponsored enterprises mortgage-backed securities | Fair Value, Measurements, Recurring | Level 2
Assets
Investment securities	204,385	10,745
Non-government mortgage-backed securities
Assets
Investment securities		157
Non-government mortgage-backed securities | Fair Value, Measurements, Recurring
Assets
Investment securities		157
Non-government mortgage-backed securities | Fair Value, Measurements, Recurring | Level 2
Assets
Investment securities		157
Non-government asset-backed securities
Assets
Investment securities	12,583	95,310
Non-government asset-backed securities | Fair Value, Measurements, Recurring
Assets
Investment securities	12,583	95,310
Non-government asset-backed securities | Fair Value, Measurements, Recurring | Level 2
Assets
Investment securities	12,583	95,310
Collateralized mortgage obligations: Government agency
Assets
Investment securities	3,351,822	2,239,934
Collateralized mortgage obligations: Government agency | Fair Value, Measurements, Recurring
Assets
Investment securities	3,351,822	2,239,934
Collateralized mortgage obligations: Government agency | Fair Value, Measurements, Recurring | Level 2
Assets
Investment securities	3,351,822	2,239,934
Collateralized mortgage obligations: Government-sponsored enterprises
Assets
Investment securities	687,921	1,029,337
Collateralized mortgage obligations: Government-sponsored enterprises | Fair Value, Measurements, Recurring
Assets
Investment securities	687,921	1,029,337
Collateralized mortgage obligations: Government-sponsored enterprises | Fair Value, Measurements, Recurring | Level 2
Assets
Investment securities	$ 687,921	$ 1,029,337