Investments Investments - Valuation Changes Included in Net Income for Investments (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Total valuation changes	$ 29
Limited partnership interests
Schedule of Available-for-sale Securities [Line Items]
Total valuation changes	38
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Total valuation changes	$ (9)