Investment Securities - Summary of Unrealized Losses on Held to Maturity Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015 Investment		Dec. 31, 2014
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 7,816,797		$ 7,421,042
Unrealized Gains	60,668		16,314
Unrealized Losses	(7,812)		(21,700)
Fair Value	7,869,653		7,415,656
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	897,720		3,438,894
Unrealized Losses, Less than 12 months	(7,812)		(16,520)
Fair Value of Investments, 12 months or longer	0	[1]	783,558	[2]
Unrealized Losses, 12 months or longer	0	[1]	(5,180)	[2]
Fair Value of Investments	897,720		4,222,452
Unrealized Losses	(7,812)		(21,700)
Number of held-to-maturity investments with unrealized loss	120
Number of held-to-maturity investments in continuous loss more than 12 months	0
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	473,373	[3]	405,899	[3]
Unrealized Gains	8,700	[3]	4,589	[3]
Unrealized Losses	0	[3]	(38)	[3]
Fair Value	482,073	[3]	410,450	[3]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months			48,335
Unrealized Losses, Less than 12 months			(38)
Fair Value of Investments, 12 months or longer			0
Unrealized Losses, 12 months or longer			0
Fair Value of Investments			48,335
Unrealized Losses			(38)
Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,693,488		2,799,923
Unrealized Gains	23,143		5,789
Unrealized Losses	(501)		(2,320)
Fair Value	2,716,130		2,803,392
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	52,761		999,230
Unrealized Losses, Less than 12 months	(501)		(2,320)
Fair Value of Investments, 12 months or longer	0		0
Unrealized Losses, 12 months or longer	0		0
Fair Value of Investments	52,761		999,230
Unrealized Losses	(501)		(2,320)
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	888,823		814,589
Unrealized Gains	9,023		1,026
Unrealized Losses	(498)		(3,800)
Fair Value	897,348		811,815
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	164,668		629,840
Unrealized Losses, Less than 12 months	(498)		(3,800)
Fair Value of Investments, 12 months or longer	0		0
Unrealized Losses, 12 months or longer	0		0
Fair Value of Investments	164,668		629,840
Unrealized Losses	(498)		(3,800)
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	83,278		83,942
Unrealized Gains	126		18
Unrealized Losses	(480)		(657)
Fair Value	82,924		83,303
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	48,864		79,141
Unrealized Losses, Less than 12 months	(480)		(657)
Fair Value of Investments, 12 months or longer	0		0
Unrealized Losses, 12 months or longer	0		0
Fair Value of Investments	48,864		79,141
Unrealized Losses	(480)		(657)
Fixed rate | Agency-issued collateralized mortgage obligations | Residential mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	3,553,640		3,185,109
Unrealized Gains	19,339		4,521
Unrealized Losses	(6,328)		(14,885)
Fair Value	3,566,651		3,174,745
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	622,767		1,682,348
Unrealized Losses, Less than 12 months	(6,328)		(9,705)
Fair Value of Investments, 12 months or longer	0		783,558
Unrealized Losses, 12 months or longer	0		(5,180)
Fair Value of Investments	622,767		2,465,906
Unrealized Losses	(6,328)		(14,885)
Variable rate | Agency-issued collateralized mortgage obligations | Residential mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	124,195		131,580
Unrealized Gains	337		371
Unrealized Losses	(5)		0
Fair Value	124,527		131,951
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Fair Value of Investments, Less than 12 months	8,660
Unrealized Losses, Less than 12 months	(5)
Fair Value of Investments, 12 months or longer	0
Unrealized Losses, 12 months or longer	0
Fair Value of Investments	8,660
Unrealized Losses	$ (5)

[1] As of March 31, 2015, we identified a total of 120 investments that were in unrealized loss positions, none of which have been in an impaired position for a period of time greater than 12 months. As of March 31, 2015, 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 March 31, 2015, 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.