Borrowings - Maturities (Details) - USD ($)	Mar. 31, 2016	Dec. 31, 2015
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 2,056,048,000	$ 1,909,292,000
Repurchase agreements	$ 1,939,489,000	$ 1,798,813,000
Weighted Average Borrowing Rate	0.69%	0.64%
Overnight (1 Day or Less) [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 243,551,000	$ 0
Repurchase agreements	$ 231,384,000	$ 0
Weighted Average Borrowing Rate	0.66%	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	$ 1,688,958,000	$ 1,894,491,000
Repurchase agreements	$ 1,591,558,000	$ 1,789,338,000
Weighted Average Borrowing Rate	0.69%	0.64%
Between 31 and 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 123,539,000	$ 14,801,000
Repurchase agreements	$ 116,547,000	$ 9,475,000
Weighted Average Borrowing Rate	0.72%	1.19%
Greater Than 90 days [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%