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Mortgage Backed Securities Mortgage backed securities designated as AFS (Details) (USD $) In Thousands, unless otherwise specified | Sep. 30, 2013 | Dec. 31, 2012
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Schedule of Available-for-sale Securities [Line Items] |  |
Investment Owned, Balance, Principal Amount | $ 3,380,838 | $ 3,181,586
Investments, Unamortized Premium (Discount), Net | 754,316 | 800,080
Available-for-sale Securities, Amortized Cost Basis | 4,135,154 | 3,981,666
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax | (22,388) | 92,246
Mortgage-backed Securities Available-for-sale, Fair Value | 4,112,766 | 4,073,912
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investment Owned, Balance, Principal Amount | 2,984,524 | 2,706,428
Investments, Unamortized Premium (Discount), Net | 648,708 | 709,246
Available-for-sale Securities, Amortized Cost Basis | 3,633,232 | 3,415,674
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax | (35,352) | 46,916
Mortgage-backed Securities Available-for-sale, Fair Value | 3,597,880 | 3,462,590
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Residential Mortgage Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investment Owned, Balance, Principal Amount | 2,723,084 | 2,425,826
Investments, Unamortized Premium (Discount), Net | 162,370 | 137,168
Available-for-sale Securities, Amortized Cost Basis | 2,885,454 | 2,562,994
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax | (56,407) | 8,343
Mortgage-backed Securities Available-for-sale, Fair Value | 2,829,047 | 2,571,337
Investments, Weighted Average Coupon Rate | 3.26% | 3.67%
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Commercial Mortgage Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investment Owned, Balance, Principal Amount | 261,440 | 280,602
Investments, Unamortized Premium (Discount), Net | 19,844 | 21,907
Available-for-sale Securities, Amortized Cost Basis | 281,284 | 302,509
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax | 11,941 | 21,564
Mortgage-backed Securities Available-for-sale, Fair Value | 293,225 | 324,073
Investments, Weighted Average Coupon Rate | 4.82% | 5.19%
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Commercial Mortgage Backed Securities, Interest Only [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investment Owned, Balance, Principal Amount | 0 | 0
Notional balance for interest only securities | 9,682,065 | 10,059,495
Investments, Unamortized Premium (Discount), Net | 466,494 | 550,171
Available-for-sale Securities, Amortized Cost Basis | 466,494 | 550,171
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax | 9,114 | 17,009
Mortgage-backed Securities Available-for-sale, Fair Value | 475,608 | 567,180
Investments, Weighted Average Coupon Rate | 0.89% | 0.95%
Mortgage-backed Securities, Issued by Private Enterprises [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investment Owned, Balance, Principal Amount | 396,314 | 475,158
Investments, Unamortized Premium (Discount), Net | 105,608 | 90,834
Available-for-sale Securities, Amortized Cost Basis | 501,922 | 565,992
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax | 12,964 | 45,330
Mortgage-backed Securities Available-for-sale, Fair Value | 514,886 | 611,322
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Residential Mortgage Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investment Owned, Balance, Principal Amount | 14,972 | 11,411
Investments, Unamortized Premium (Discount), Net | (351) | (781)
Available-for-sale Securities, Amortized Cost Basis | 14,621 | 10,630
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax | (33) | 408
Mortgage-backed Securities Available-for-sale, Fair Value | 14,588 | 11,038
Investments, Weighted Average Coupon Rate | 4.58% | 4.28%
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Commercial Mortgage Backed Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investment Owned, Balance, Principal Amount | 381,342 | 463,747
Investments, Unamortized Premium (Discount), Net | (17,213) | (17,313)
Available-for-sale Securities, Amortized Cost Basis | 364,129 | 446,434
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax | 10,620 | 39,908
Mortgage-backed Securities Available-for-sale, Fair Value | 374,749 | 486,342
Investments, Weighted Average Coupon Rate | 5.52% | 5.31%
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Commercial Mortgage Backed Securities, Interest Only [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Investment Owned, Balance, Principal Amount | 0 | 0
Notional balance for interest only securities | 2,857,343 | 2,393,614
Investments, Unamortized Premium (Discount), Net | 123,172 | 108,928
Available-for-sale Securities, Amortized Cost Basis | 123,172 | 108,928
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax | 2,377 | 5,014
Mortgage-backed Securities Available-for-sale, Fair Value | 125,549 | 113,942
Investments, Weighted Average Coupon Rate | 0.87% | 0.86%
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 2,129,694 | 1,026,277
Available-for-sale Securities, Continuous Loss Position Less Than 12 Months, Aggregate Losses | (48,337) | (6,552)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | 164 | 83
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 558,070 | 271,719
Available-for-sale Securities, Continuous Unrealized Loss Position for 12 Months or Longer, Aggregate Losses | (17,718) | (5,797)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | 67 | 34
Mortgage-backed Securities, Issued by Private Enterprises [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 126,846 | 13,877
Available-for-sale Securities, Continuous Loss Position Less Than 12 Months, Aggregate Losses | (5,902) | (76)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | 23 | 3
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 1,820 | 2,701
Available-for-sale Securities, Continuous Unrealized Loss Position for 12 Months or Longer, Aggregate Losses | $ (100) | $ (198)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | 6 | 8

X
- Definition Amount of accumulated pre-tax unrealized loss on investments in available-for-sale securities that have been in a loss position for less than 12 months. + References No definition available. + Details
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- Definition Amount of accumulated pre-tax unrealized loss on securities classified as available-for-sale that have been in a loss position for twelve months or longer. + References No definition available. + Details
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- Definition The remaining amount of investment premium (discount), net, originally recognized at the purchase date of the instrument that has yet to be amortized (accreted). + References No definition available. + Details
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- Definition This item represents the current weighted-average gross interest rate earned on this type of MBS in the investment portfolio as of the period indicated. The gross interest rate of each MBS within the portfolio is weighted based on its current outstanding notional or par balance. The current outstanding notional or par balance for each MBS consists of the total of all principal balances remaining on the mortgage loans underlying each MBS. + References No definition available. + Details
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- Definition Notional balance for interest only securities owned by the Company. Interest only securities represent the right to receive excess interest payments and not principal cash flows based on the underlying unpaid principal balance (often referred to as the notional amount) of the underlying pool of mortgage loans. + References No definition available. + Details
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- Definition Accumulated appreciation or loss, net of tax, in value of the total of available-for-sale securities at the end of an accounting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=28358780&loc=SL7669686-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=28358780&loc=d3e637-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=28358780&loc=d3e681-108580 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=28358780&loc=SL7669646-108580 + Details
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- Definition This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References No definition available. + Details
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- Definition This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563 + Details
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- Definition Number of investment positions in available-for-sale investments in a continuous unrealized loss position for less than one year or normal operating cycle, if longer, for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement. + References No definition available. + Details
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