Repurchase Agreements (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Repurchase agreement, balance	$ 3,674,850	$ 3,564,128
Repurchage agreement, Weighted Average Interest Rate	0.59%	0.70%
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	4,008,997	3,935,504
Deferred Costs	(347)	(659)
Weighted Average Maturity of Agreements	91 days	57 days
Bonds [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Repurchase agreement, balance	21,403	28,113
Repurchage agreement, Weighted Average Interest Rate	1.60%	1.64%
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	24,853	31,483
Residential Mortgage Backed Securities [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Repurchase agreement, balance	2,634,120	2,365,982
Repurchage agreement, Weighted Average Interest Rate	0.41%	0.48%
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	2,721,562	2,458,200
Residential Mortgage Backed Securities [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Repurchase agreement, balance	10,932	7,808
Repurchage agreement, Weighted Average Interest Rate	1.80%	1.84%
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	13,616	9,634
Commercial Mortgage Backed Securities [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Repurchase agreement, balance	234,633	248,771
Repurchage agreement, Weighted Average Interest Rate	0.39%	0.47%
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	294,556	291,445
Commercial Mortgage Backed Securities [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Repurchase agreement, balance	298,548	382,352
Repurchage agreement, Weighted Average Interest Rate	1.29%	1.34%
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	364,977	465,306
Commercial Mortgage Backed Securities, Interest Only [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Repurchase agreement, balance	377,977	443,540
Repurchage agreement, Weighted Average Interest Rate	1.17%	1.22%
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	463,894	565,494
Commercial Mortgage Backed Securities, Interest Only [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Repurchase agreement, balance	97,584	88,221
Repurchage agreement, Weighted Average Interest Rate	1.68%	1.46%
Pledged Financial Instruments, Not Separately Reported, Mortgage-Related Securities Available-for-sale or Held-for-investment	$ 125,539	$ 113,942