Derivative Financial Instruments - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Derivative [Line Items]
Cash flow hedge losses to be reclassified from AOCI during the next twelve months (less than)	$ 1,000,000		$ 1,000,000
Net gain (loss) recognized in earnings during the reporting period representing the amount of the fair value of the hedges' ineffectiveness.	0	$ 0	0	$ 0
Cash and securities pledged as collateral by counterparties	24,000,000		24,000,000
Securities pledged as collateral to counterparties	3,000,000		3,000,000
Collateral posted under MNAs for contracts containing credit-risk-contingent features	1,000,000		1,000,000		$ 6,000,000
Maximum amount of additional exposure for contracts with credit-risk-contingent features if all features were triggered concurrently	2,000,000		2,000,000
Cash and securities pledged in the form of margin deposits	16,000,000		$ 16,000,000
Term of credit default swaps (in years)			5 years
Counterparties
Derivative [Line Items]
Collateral posted under MNAs for contracts containing credit-risk-contingent features	$ 1,000,000		$ 1,000,000