Securities (Details 1) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities held to maturity
Estimated Fair Market Value	$ 4,153,599
Mortgage-backed securities [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities held to maturity
Amortized Cost	3,853,733	4,115,523
Gross Unrealized Gains	254,557	252,904
Gross Unrealized Losses	(3)	(4)
Estimated Fair Market Value	4,108,287	4,368,423
Mortgage-backed securities [Member] | GNMA pass-through certificates [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities held to maturity
Amortized Cost	80,987	83,587
Gross Unrealized Gains	2,784	2,602
Gross Unrealized Losses
Estimated Fair Market Value	83,771	86,189
Mortgage-backed securities [Member] | FNMA pass-through certificates [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities held to maturity
Amortized Cost	1,082,570	1,154,638
Gross Unrealized Gains	79,020	78,603
Gross Unrealized Losses	(3)	(4)
Estimated Fair Market Value	1,161,587	1,233,237
Mortgage-backed securities [Member] | FHLMC pass-through certificates [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities held to maturity
Amortized Cost	2,018,619	2,132,408
Gross Unrealized Gains	127,813	125,364
Gross Unrealized Losses
Estimated Fair Market Value	2,146,432	2,257,772
Mortgage-backed securities [Member] | FHLMC and FNMA - REMICs [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities held to maturity
Amortized Cost	671,557	744,890
Gross Unrealized Gains	44,940	46,335
Gross Unrealized Losses
Estimated Fair Market Value	716,497	791,225
Investment Securities [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities held to maturity
Amortized Cost	39,011	539,011
Gross Unrealized Gains	6,301	6,750
Gross Unrealized Losses
Estimated Fair Market Value	45,312	545,761
Investment Securities [Member] | United States government-sponsored enterprises Debt [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities held to maturity
Amortized Cost	39,011	539,011
Gross Unrealized Gains	6,301	6,750
Gross Unrealized Losses
Estimated Fair Market Value	$ 45,312	$ 545,761