Nature of Operations and Significant Accounting Policies - Adoption of "Revenue Recognition" Guidance (Details) - USD ($) $ in Millions	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Assets
Deferred acquisition costs	$ 3,467.9		$ 3,540.7
Other assets	1,522.2		1,330.9
Liabilities
Deferred income taxes	909.0		1,092.5
Stockholders' equity
Total stockholders' equity	12,255.8		$ 12,921.9	$ 10,684.9	$ 10,293.8
ASU 2014-09 - Revenue recognition | As adjusted
Assets
Deferred acquisition costs	3,671.5
Other assets	1,380.9
Liabilities
Deferred income taxes	931.2
Stockholders' equity
Total stockholders' equity	12,295.9
ASU 2014-09 - Revenue recognition | Impact of adopting revenue recognition accounting guidance
Assets
Deferred acquisition costs	(203.6)
Other assets	141.3
Liabilities
Deferred income taxes	(22.2)
Stockholders' equity
Total stockholders' equity	$ (40.1)	$ (39.7)