Financial Derivatives - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Derivative [Line Items]
Gain (Loss) on Discontinuation of Interest Rate Cash Flow Hedge Due to Forecasted Transaction Probable of Not Occurring, Net	$ 0	$ 0	$ 0	$ 0
Securities Purchased under Agreements to Resell	0		0		$ 0
Financial derivatives, at fair value	140,758		140,758		77,199
Derivative, Collateral [Abstract]
Collateral Pledged	95,144		95,144		37,986
Additional Collateral, Aggregate Fair Value	40,000		40,000		45,200
Notional Disclosures [Abstract]
Derivative, Notional Amount	8,532,790		8,532,790		8,429,964
Excluding netting [Member] | Including accrued interest [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	15,600		15,600		6,400
Financial derivatives, at fair value	152,300		152,300		90,100
Netting [Member] | Including accrued interest [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	0		0		0
Financial derivatives, at fair value	135,100		135,100		83,200
Uncollateralized [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	0		0		0
Cash [Member]
Derivative, Collateral [Abstract]
Collateral Received	0		0		0
Collateral Pledged	95,100		95,100		38,000
Securities Pledged as Collateral [Member]
Derivative, Collateral [Abstract]
Collateral Pledged	0		0		0
Exchange Cleared [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	7,500,000		7,500,000		6,200,000
Interest Rate Swap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 8,500,000		$ 8,500,000		$ 8,400,000