Borrowings - Narrative (Details) - USD ($)	Mar. 31, 2016	Dec. 31, 2015
Disclosure of Repurchase Agreements [Abstract]
Repurchase agreements	$ 1,939,489,000	$ 1,798,813,000
Fair Value of securities pledged, including accrued interest receivable	2,056,048,000	1,909,292,000
Cash Held on Deposit with Repurchase Agreement Counterparties	$ 1,250,000	$ 3,997,000
Weighted Average Borrowing Rate	0.69%	0.64%
Maximum Amount at Risk	$ 116,200,000
FHLB Advances	$ 0	$ 187,500,000
Weighted Average Borrowing Rate - FHLB Advances		0.42%
Collateral Held On FHLB Advances		$ 192,200,000
FHLBC Stock		$ 3,800,000