Investments (Unrealized Losses) (Details)	3 Months Ended
	Mar. 31, 2018 USD ($) security
Asset-Backed Security
Available-for-Sale Securities and Held-to-Maturity Securities [Line Items]
Securities, other-than-temporarily impaired | $	$ 0
Debt Securities
Available-for-Sale Securities and Held-to-Maturity Securities [Line Items]
Gross unrealized losses, number of issues	531
Gross unrealized losses, percentage of total debt	40.10%
Gross unrealized losses, market value as a percent of amortized cost	97.60%
Gross unrealized losses, number of securities with maturities of 12 months or greater	120
Gross unrealized losses, number of securities with maturities of 12 months or greater (as a percentage)	22.60%
External Credit Rating, Investment Grade | Debt Securities
Available-for-Sale Securities and Held-to-Maturity Securities [Line Items]
Gross unrealized losses, number of securities rated investment grade	521