Investment Securities - Summary of Unrealized Losses on Held to Maturity Securities (Details) (USD $)	Jun. 30, 2014 Investment	Jun. 01, 2014	Dec. 31, 2013	Jun. 30, 2013
Schedule of Held-to-maturity Securities [Line Items]
Adjusted cost of transferred securities		$ 5,400,000,000
Unrealized gains for transferred securities		22,500,000
Amortized Cost	5,463,920,000	5,418,572,000		0
Unrealized Gains	4,894,000
Unrealized Losses	(13,818,000)
Fair Value	5,454,996,000		0
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	3,377,237,000
Unrealized Losses, Less than 12 months	(6,808,000)
Fair Value of Investments, 12 months or longer	919,376,000
Unrealized Losses, 12 months or longer	(7,010,000)
Fair Value of Investments	4,296,613,000
Unrealized Losses	(13,818,000)
Number of held-to-maturity investments with unrealized loss	276
Number of held-to-maturity investments in continuous loss more than 12 months	31
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	318,260,000
Unrealized Gains	1,244,000
Unrealized Losses	(165,000)
Fair Value	319,339,000
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	171,001,000
Unrealized Losses, Less than 12 months	(105,000)
Fair Value of Investments, 12 months or longer	48,590,000
Unrealized Losses, 12 months or longer	(60,000)
Fair Value of Investments	219,591,000
Unrealized Losses	(165,000)
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	550,269,000
Unrealized Gains	2,149,000
Unrealized Losses	(515,000)
Fair Value	551,903,000
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	268,818,000
Unrealized Losses, Less than 12 months	(515,000)
Fair Value of Investments, 12 months or longer	0
Unrealized Losses, 12 months or longer	0
Fair Value of Investments	268,818,000
Unrealized Losses	(515,000)
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	85,943,000
Unrealized Gains	16,000
Unrealized Losses	(207,000)
Fair Value	85,752,000
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	69,524,000
Unrealized Losses, Less than 12 months	(207,000)
Fair Value of Investments, 12 months or longer	0
Unrealized Losses, 12 months or longer	0
Fair Value of Investments	69,524,000
Unrealized Losses	(207,000)
Agency-issued collateralized mortgage obligations | Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	3,051,644,000
Unrealized Gains	847,000
Unrealized Losses	(3,991,000)
Fair Value	3,048,500,000
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	2,473,332,000
Unrealized Losses, Less than 12 months	(3,945,000)
Fair Value of Investments, 12 months or longer	20,180,000
Unrealized Losses, 12 months or longer	(46,000)
Fair Value of Investments	2,493,512,000
Unrealized Losses	(3,991,000)
Variable rate | Agency-issued collateralized mortgage obligations | Residential mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	148,422,000
Unrealized Gains	19,000
Unrealized Losses	(1,000)
Fair Value	148,440,000
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	10,799,000
Unrealized Losses, Less than 12 months	(1,000)
Fair Value of Investments, 12 months or longer	0
Unrealized Losses, 12 months or longer	0
Fair Value of Investments	10,799,000
Unrealized Losses	(1,000)
Fixed rate | Agency-issued collateralized mortgage obligations | Residential mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,309,382,000
Unrealized Gains	619,000
Unrealized Losses	(8,939,000)
Fair Value	1,301,062,000
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	383,763,000
Unrealized Losses, Less than 12 months	(2,035,000)
Fair Value of Investments, 12 months or longer	850,606,000
Unrealized Losses, 12 months or longer	(6,904,000)
Fair Value of Investments	1,234,369,000
Unrealized Losses	$ (8,939,000)