Fair Value - Assets and Liabilities Recorded at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Assets
Investment securities	$ 4,601,267	$ 4,027,265
Fair Value, Measurements, Recurring
Assets
Investment securities	4,601,267	4,027,265
Other assets	31,409	11,002
Liabilities
Other liabilities	(54,570)	(23,122)
Total
Net Assets (Liabilities)	4,578,106	4,015,145
Fair Value, Measurements, Recurring | Level 2
Assets
Investment securities	4,601,267	4,027,265
Other assets	31,409	11,002
Liabilities
Other liabilities	(46,194)	(23,122)
Total
Net Assets (Liabilities)	4,586,482	4,015,145
Fair Value, Measurements, Recurring | Level 3
Liabilities
Other liabilities	(8,376)
Total
Net Assets (Liabilities)	(8,376)
U.S. Treasury securities
Assets
Investment securities		499,976
U.S. Treasury securities | Fair Value, Measurements, Recurring
Assets
Investment securities		499,976
U.S. Treasury securities | Fair Value, Measurements, Recurring | Level 2
Assets
Investment securities		499,976
Non-government securities
Assets
Investment securities	159,408	95,824
Non-government securities | Fair Value, Measurements, Recurring
Assets
Investment securities	159,408	95,824
Non-government securities | Fair Value, Measurements, Recurring | Level 2
Assets
Investment securities	159,408	95,824
Government agency mortgage-backed securities
Assets
Investment securities	192,178	55,982
Government agency mortgage-backed securities | Fair Value, Measurements, Recurring
Assets
Investment securities	192,178	55,982
Government agency mortgage-backed securities | Fair Value, Measurements, Recurring | Level 2
Assets
Investment securities	192,178	55,982
Government-sponsored enterprises mortgage-backed securities
Assets
Investment securities	9,515	10,745
Government-sponsored enterprises mortgage-backed securities | Fair Value, Measurements, Recurring
Assets
Investment securities	9,515	10,745
Government-sponsored enterprises mortgage-backed securities | Fair Value, Measurements, Recurring | Level 2
Assets
Investment securities	9,515	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	36,089	95,310
Non-government asset-backed securities | Fair Value, Measurements, Recurring
Assets
Investment securities	36,089	95,310
Non-government asset-backed securities | Fair Value, Measurements, Recurring | Level 2
Assets
Investment securities	36,089	95,310
Collateralized mortgage obligations: Government agency
Assets
Investment securities	3,134,301	2,239,934
Collateralized mortgage obligations: Government agency | Fair Value, Measurements, Recurring
Assets
Investment securities	3,134,301	2,239,934
Collateralized mortgage obligations: Government agency | Fair Value, Measurements, Recurring | Level 2
Assets
Investment securities	3,134,301	2,239,934
Collateralized mortgage obligations: Government-sponsored enterprises
Assets
Investment securities	1,069,776	1,029,337
Collateralized mortgage obligations: Government-sponsored enterprises | Fair Value, Measurements, Recurring
Assets
Investment securities	1,069,776	1,029,337
Collateralized mortgage obligations: Government-sponsored enterprises | Fair Value, Measurements, Recurring | Level 2
Assets
Investment securities	$ 1,069,776	$ 1,029,337