Financial Derivatives - Narrative (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	$ 1,900,000
Gain (Loss) on Discontinuation of Interest Rate Cash Flow Hedge Due to Forecasted Transaction Probable of Not Occurring, Net	0
Securities Purchased under Agreements to Resell	0
Financial derivatives, at fair value	17,841,000	$ 26,599,000
Derivative, Collateral [Abstract]
Collateral Pledged	46,841,000	24,926,000
Additional Collateral, Aggregate Fair Value	0	3,100,000
Notional Disclosures [Abstract]
Derivative, Notional Amount	9,703,115,000	8,860,296,000
Uncollateralized [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	4,400,000	500,000
Cash [Member]
Derivative, Collateral [Abstract]
Collateral Received	0	0
Collateral Pledged	0	100,000
Securities Pledged as Collateral [Member]
Derivative, Collateral [Abstract]
Collateral Pledged	46,800,000	24,800,000
Exchange Cleared [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	8,300,000,000	7,900,000,000
Interest Rate Swap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	9,700,000,000	8,800,000,000
Including accrued interest [Member] | Excluding netting [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	94,600,000	28,500,000
Financial derivatives, at fair value	50,100,000	58,200,000
Including accrued interest [Member] | Netting [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	4,400,000	500,000
Financial derivatives, at fair value	$ 6,400,000	$ 28,000,000