Litigation and Contingencies (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Average paid claim reduction due to curtailments (as a percent)	6.30%	5.60%
Maximum exposure associated with other discussions and legal proceedings	$ 286