Repurchase Agreements - Maturities (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 1,552,735,000	$ 1,522,905,000
Repurchase agreements	1,459,490,000	1,436,651,000
Weighted Average Borrowing Rate	0.36%	0.36%
Overnight (1 Day or Less) [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	0	0
Repurchase agreements	0	0
Weighted Average Borrowing Rate	0.00%	0.00%
Between 2 and 30 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	1,297,828,000	984,823,000
Repurchase agreements	1,222,961,000	929,831,000
Weighted Average Borrowing Rate	0.36%	0.36%
Between 31 and 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	254,907,000	534,238,000
Repurchase agreements	236,529,000	502,947,000
Weighted Average Borrowing Rate	0.38%	0.37%
Greater Than 90 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	0	3,844,000
Repurchase agreements	$ 0	$ 3,873,000
Weighted Average Borrowing Rate	0.00%	0.38%