Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Unbilled Audit Premiums		$ 8	$ 8	$ 3
Deposit contracts, liabilities		47	51
Depreciation		$ 50	$ 47	$ 45
Indefinite-lived Intangible Assets [Line Items]
Basic weighted average number of shares outstanding (in shares)		124,843,240	122,650,997	124,040,313
Interest payments		$ 145	$ 137	$ 130
Income taxes paid		207	232	$ 165
Other Assets
Indefinite-lived Intangible Assets [Line Items]
Intangible assets, net (excluding goodwill)		$ 107	$ 82
Grantor Trust | Restricted Stock Units (RSUs)
Indefinite-lived Intangible Assets [Line Items]
Basic weighted average number of shares outstanding (in shares)	4,847,303