Financial Derivatives - Narrative (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Securities Purchased under Agreements to Resell	$ 0	$ 0
Financial derivatives, at fair value	117,956	77,199
Derivative, Collateral [Abstract]
Collateral Pledged	73,659	37,986
Additional Collateral, Aggregate Fair Value	39,500	45,200
Notional Disclosures [Abstract]
Derivative, Notional Amount	8,949,663	8,429,964
Excluding netting [Member] | Including accrued interest [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	12,600	6,400
Financial derivatives, at fair value	126,700	90,100
Netting [Member] | Including accrued interest [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	200	0
Financial derivatives, at fair value	113,200	83,200
Uncollateralized [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	200	0
Cash [Member]
Derivative, Collateral [Abstract]
Collateral Received	0	0
Collateral Pledged	73,700	38,000
Securities Pledged as Collateral [Member]
Derivative, Collateral [Abstract]
Collateral Pledged	0	0
Exchange Cleared [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	7,400,000	6,200,000
Interest Rate Swap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 8,900,000	$ 8,400,000