Financial Derivatives - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Derivative [Line Items]
Financial derivatives, at fair value	$ 94,880		$ 94,880		$ 84,844
Derivative, Collateral [Abstract]
Collateral Pledged	48,111		48,111		46,627
Additional Collateral, Aggregate Fair Value	44,900		44,900		46,800
Derivative, Net Liability Position, Aggregate Fair Value	0		0		0
Notional Disclosures [Abstract]
Derivative, Notional Amount	8,052,133		8,052,133		6,837,571
Excluding netting [Member] | Including accrued interest [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	12,400		12,400		6,100
Financial derivatives, at fair value	105,700		105,700		99,400
Netting [Member] | Including accrued interest [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	200		200		400
Financial derivatives, at fair value	93,000		93,000		93,400
Uncollateralized [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	200		200		400
Cash [Member]
Derivative, Collateral [Abstract]
Collateral Received	0		0		0
Collateral Pledged	48,100		48,100		46,600
Securities Pledged as Collateral [Member]
Derivative, Collateral [Abstract]
Collateral Pledged	0		0		0
Exchange Cleared [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	5,700,000		5,700,000		4,000,000
Interest Rate Swap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	8,000,000		8,000,000		$ 6,800,000
Fair Value Hedging [Member] | Interest Rate Swap [Member]
Notional Disclosures [Abstract]
Gain (Loss) on Fair Value Hedge Ineffectiveness, Net	$ 300	$ 200	$ (380)	$ 300