Derivative Financial Instruments - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Term of credit default swaps (in years)	5 years
Cash flow hedge losses to be reclassified from AOCI during the next twelve months	$ 2,000,000
Net gain (loss) recognized in earnings during the reporting period representing the amount of the fair value of the hedges' ineffectiveness.	0	$ 0
Cash and securities pledged as collateral by counterparties	8,000,000
Securities pledged as collateral to counterparties	21,000,000
Collateral posted under MNAs for contracts containing credit-risk-contingent features	4,000,000		$ 6,000,000
Maximum amount of additional exposure for contracts with credit-risk-contingent features if all features were triggered concurrently	17,000,000
Cash and securities pledged in the form of margin deposits	$ 15,000,000