Recently Issued Accounting Standards (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Unrealized gain on investments, net of income taxes	$ 22.5	$ 18.6
Service cost	7.5	8.0
Other components of net benefit cost	$ 1.7	$ (4.6)