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Subsequent Event (Details) - USD ($) $ in Millions	Feb. 26, 2019	Feb. 14, 2019	Feb. 13, 2019	Dec. 31, 2018	Dec. 31, 2017
MediaAlpha
Subsequent Event [Line Items]
Ownership interest (as a percent)				61.00%	64.40%
MediaAlpha \| Subsequent Event
Subsequent Event [Line Items]
Assets, fair value	$ 350
Ownership interest (as a percent)	42.00%
Equity in earnings of unconsolidated affiliates, net of tax	$ 88
Kudu \| Subsequent Event
Subsequent Event [Line Items]
Ownership interest (as a percent)		99.00%	49.50%
Equity method investment, aggregate cost		$ 50
Unfunded capital commitment		$ 167