Financial Derivatives - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Derivative [Line Items]
Gain expected to be reclassified from AOCI to earnings over next 12 months	$ 800,000		$ 800,000
Gain (loss) from interest rate swaps designated as cash flow hedges reclassified to earnings	0	$ 0	0	$ 0
Investments held as collateral	0		0		$ (1,100,000)
Cash posted as collateral	167,060,000		167,060,000		47,018,000
Financial derivatives in net payable position	0		0		0
Notional amount	13,799,053,000		13,799,053,000		9,955,745,000
Uncollateralized [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	(300,000)		(300,000)		1,400,000
Cash [Member]
Derivative [Line Items]
Cash held as collateral	(2,700,000)		(2,700,000)		(700,000)
Cash posted as collateral	600,000		600,000		0
Available-for-sale Securities [Member]
Derivative [Line Items]
Cash posted as collateral					47,000,000.0
Exchange Cleared [Member]
Derivative [Line Items]
Notional amount	10,800,000,000		10,800,000,000		8,500,000,000
Interest Rate Swap [Member]
Derivative [Line Items]
Notional amount	$ 13,800,000,000		$ 13,800,000,000		$ 9,900,000,000