Investment Securities - Summary of Unrealized Losses on Held to Maturity Securities (Details) (USD $)	Sep. 30, 2014 Investment		Jun. 01, 2014	Dec. 31, 2013	Sep. 30, 2013
Schedule of Held-to-maturity Securities [Line Items]
Adjusted cost of transferred securities	$ 5,418,572,000		$ 5,400,000,000
Unrealized gains for transferred securities			22,500,000
Amortized Cost	6,662,025,000				0
Unrealized Gains	2,426,000
Unrealized Losses	(50,558,000)
Fair Value	6,613,893,000			0
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	5,123,941,000	[1]
Unrealized Losses, Less than 12 months	(38,616,000)	[1]
Fair Value of Investments, 12 months or longer	854,029,000	[1],[2]
Unrealized Losses, 12 months or longer	(11,942,000)	[1],[2]
Fair Value of Investments	5,977,970,000	[1]
Unrealized Losses	(50,558,000)	[1]
Number of held-to-maturity investments with unrealized loss	329
Number of held-to-maturity investments in continuous loss more than 12 months	28
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	349,993,000	[3]
Unrealized Gains	1,588,000	[3]
Unrealized Losses	(1,165,000)	[3]
Fair Value	350,416,000	[3]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	205,383,000
Unrealized Losses, Less than 12 months	(837,000)
Fair Value of Investments, 12 months or longer	47,987,000	[2]
Unrealized Losses, 12 months or longer	(328,000)	[2]
Fair Value of Investments	253,370,000
Unrealized Losses	(1,165,000)
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	679,379,000
Unrealized Gains	350,000
Unrealized Losses	(4,391,000)
Fair Value	675,338,000
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	578,687,000
Unrealized Losses, Less than 12 months	(4,391,000)
Fair Value of Investments, 12 months or longer	0	[2]
Unrealized Losses, 12 months or longer	0	[2]
Fair Value of Investments	578,687,000
Unrealized Losses	(4,391,000)
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	84,218,000
Unrealized Gains	81,000
Unrealized Losses	(224,000)
Fair Value	84,075,000
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	55,365,000
Unrealized Losses, Less than 12 months	(224,000)
Fair Value of Investments, 12 months or longer	0	[2]
Unrealized Losses, 12 months or longer	0	[2]
Fair Value of Investments	55,365,000
Unrealized Losses	(224,000)
Agency-issued collateralized mortgage obligations | Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,919,715,000
Unrealized Gains	16,000
Unrealized Losses	(20,712,000)
Fair Value	2,899,019,000
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	2,890,039,000
Unrealized Losses, Less than 12 months	(20,712,000)
Fair Value of Investments, 12 months or longer	0	[2]
Unrealized Losses, 12 months or longer	0	[2]
Fair Value of Investments	2,890,039,000
Unrealized Losses	(20,712,000)
Fixed rate | Agency-issued collateralized mortgage obligations | Residential mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,489,538,000
Unrealized Gains	384,000
Unrealized Losses	(24,052,000)
Fair Value	2,465,870,000
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	1,316,900,000
Unrealized Losses, Less than 12 months	(12,438,000)
Fair Value of Investments, 12 months or longer	806,042,000	[2]
Unrealized Losses, 12 months or longer	(11,614,000)	[2]
Fair Value of Investments	2,122,942,000
Unrealized Losses	(24,052,000)
Variable rate | Agency-issued collateralized mortgage obligations | Residential mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	139,182,000
Unrealized Gains	7,000
Unrealized Losses	(14,000)
Fair Value	139,175,000
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	77,567,000
Unrealized Losses, Less than 12 months	(14,000)
Fair Value of Investments, 12 months or longer	0	[2]
Unrealized Losses, 12 months or longer	0	[2]
Fair Value of Investments	77,567,000
Unrealized Losses	$ (14,000)

[1] As of September 30, 2014, we identified a total of 329 investments that were in unrealized loss positions, of which 28 investments totaling $854.0 million with unrealized losses of $11.9 million have been in an impaired position for a period of time greater than 12 months. As of September 30, 2014, we do not intend to sell any impaired fixed income investment securities prior to recovery of our adjusted cost basis, and it is more likely than not that we will not be required to sell any of our securities prior to recovery of our adjusted cost basis, which is consistent with our classification of these securities. Based on our analysis as of September 30, 2014, we deem all impairments to be temporary. Market valuations and impairment analyses on assets in the held-to-maturity securities portfolio are reviewed and monitored on a quarterly basis.

[2] Represents securities in an unrealized loss position for twelve months or longer in which the amortized cost basis was re-set for those securities re-designated from AFS to HTM effective June 1, 2014.

[3] Consists of pools of Small Business Investment Company debentures issued and guaranteed by the U.S. Small Business Administration, an independent agency of the United States.