Financial Derivatives - Narrative (Details) - USD ($)	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Derivative [Line Items]
Gain expected to be reclassified from AOCI to earnings over next 12 months	$ 300,000	$ 300,000
Gain (loss) from interest rate swaps designated as cash flow hedges reclassified to earnings	0	0	$ 0
Investments held as collateral	0	0		$ (1,100,000)
Cash posted as collateral	100,830,000	100,830,000		47,018,000
Financial derivatives in net payable position	0	0		0
Notional amount	13,186,456,000	13,186,456,000		9,955,745,000
Uncollateralized [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	44,000	44,000		1,400,000
Cash [Member]
Derivative [Line Items]
Cash held as collateral	(4,400,000)	(4,400,000)		(700,000)
Cash posted as collateral	100,000	100,000		0
Available-for-sale Securities [Member]
Derivative [Line Items]
Cash posted as collateral	100,700,000	100,700,000		47,000,000.0
Exchange Cleared [Member]
Derivative [Line Items]
Notional amount	10,700,000,000	10,700,000,000		8,500,000,000
Interest Rate Swap [Member]
Derivative [Line Items]
Notional amount	$ 13,200,000,000	$ 13,200,000,000		$ 9,900,000,000