Repurchase Agreements (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Mar. 31, 2013
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Repurchase agreement, balance	$ 3,564,128		$ 3,708,255
Repurchage agreement, Weighted Average Interest Rate	0.61%		0.65%
Fair Value of Collateral Pledged	3,935,504		4,098,351
Deferred Costs	(659)		(556)
Weighted Average Maturity of Agreements	58 days	57 days
Line of Credit Facility, Amount Outstanding			200,000
Line of Credit Facility, Interest Rate at Period End			1.47%
Bonds [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Repurchase agreement, balance	28,113		23,566
Repurchage agreement, Weighted Average Interest Rate	1.64%		1.61%
Fair Value of Collateral Pledged	31,483		26,945
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	2,365,982		2,476,620
Repurchage agreement, Weighted Average Interest Rate	0.48%		0.43%
Fair Value of Collateral Pledged	2,458,200		2,589,965
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	7,808		7,283
Repurchage agreement, Weighted Average Interest Rate	1.84%		1.85%
Fair Value of Collateral Pledged	9,634		8,289
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	248,771		245,723
Repurchage agreement, Weighted Average Interest Rate	0.47%		0.41%
Fair Value of Collateral Pledged	291,445		286,951
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	382,352		392,609
Repurchage agreement, Weighted Average Interest Rate	1.34%		1.32%
Fair Value of Collateral Pledged	465,306		475,709
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	443,540		477,290
Repurchage agreement, Weighted Average Interest Rate	1.22%		1.20%
Fair Value of Collateral Pledged	565,494		599,086
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	88,221		85,720
Repurchage agreement, Weighted Average Interest Rate	1.46%		1.47%
Fair Value of Collateral Pledged	$ 113,942		$ 111,406