Accounting Policies - Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) company	Sep. 30, 2016	Sep. 30, 2017 USD ($) company	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of insurance companies based in the United States | company	6		6
Investment in variable interest entities	$ 31.3		$ 31.3		$ 27.1
Effective U.S. federal income tax expense rate	23.80%	4.10%	11.80%	6.50%
Percentage of available for sale securities	12.20%	10.00%	12.20%	10.00%
Accounting Standards Update ("ASU") 2016-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in net cash provided by operating activities				$ 1.2
Increase in net cash used in financing activities				$ 1.2