Repurchase Agreements (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Jun. 30, 2013
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Repurchase agreement, balance	$ 3,564,128		$ 4,071,392
Repurchage agreement, Weighted Average Interest Rate	0.70%		0.60%
Fair Value of Collateral Pledged	3,935,504		4,430,811
Deferred Costs	(659)		(451)
Weighted Average Maturity of Agreements	49 days	57 days
Line of Credit Facility, Amount Outstanding			193,204
Line of Credit Facility, Interest Rate at Period End			1.45%
Bonds [Member]
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Repurchase agreement, balance	28,113		23,017
Repurchage agreement, Weighted Average Interest Rate	1.64%		1.61%
Fair Value of Collateral Pledged	31,483		25,893
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		2,885,261
Repurchage agreement, Weighted Average Interest Rate	0.48%		0.40%
Fair Value of Collateral Pledged	2,458,200		2,994,711
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		8,986
Repurchage agreement, Weighted Average Interest Rate	1.84%		1.88%
Fair Value of Collateral Pledged	9,634		10,796
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		244,739
Repurchage agreement, Weighted Average Interest Rate	0.47%		0.39%
Fair Value of Collateral Pledged	291,445		295,461
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		368,060
Repurchage agreement, Weighted Average Interest Rate	1.34%		1.32%
Fair Value of Collateral Pledged	465,306		432,212
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		439,482
Repurchage agreement, Weighted Average Interest Rate	1.22%		1.19%
Fair Value of Collateral Pledged	565,494		541,133
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		102,298
Repurchage agreement, Weighted Average Interest Rate	1.46%		1.38%
Fair Value of Collateral Pledged	$ 113,942		$ 130,605