Borrowings - Maturities (Details) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 1,909,292,000	$ 1,522,905,000
Repurchase agreements	$ 1,798,813,000	$ 1,436,651,000
Weighted Average Borrowing Rate - Repurchase Agreements	0.64%	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 - Repurchase Agreements	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	$ 1,894,491,000	$ 984,823,000
Repurchase agreements	$ 1,789,338,000	$ 929,831,000
Weighted Average Borrowing Rate - Repurchase Agreements	0.64%	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	$ 14,801,000	$ 534,238,000
Repurchase agreements	$ 9,475,000	$ 502,947,000
Weighted Average Borrowing Rate - Repurchase Agreements	1.19%	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 - Repurchase Agreements	0.00%	0.38%