Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Unbilled Audit Premiums	$ (4)	$ (4)	$ 8	$ 8
Indefinite-lived Intangible Assets [Line Items]
Basic weighted average number of shares outstanding (in shares)		126,698,927	124,843,240	122,650,997
Deposit contracts, liabilities	45	$ 45	$ 47
Depreciation		54	50	$ 47
Interest payments		155	145	137
Income taxes paid		186	207	$ 232
Other Assets
Indefinite-lived Intangible Assets [Line Items]
Intangible assets, net (excluding goodwill)	$ 104	$ 104	$ 107
Grantor Trust | Restricted Stock Units (RSUs)
Indefinite-lived Intangible Assets [Line Items]
Basic weighted average number of shares outstanding (in shares)	4,926,521