Employee Benefit Plans, Shares Available For Grant (Details) shares in Millions, $ in Millions	1 Months Ended	12 Months Ended
	May 31, 2017 shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($)
Shares Available For Grant
Beginning Balance, Number of Shares		22.5
Additional shares authorized	23.0
Ending Balance, Number of Shares			22.5
Common stock for each share subject to RSUs and PSAs (in shares)		2.1
Intrinsic value of options exercised in period | $		$ 7.1	$ 7.1	$ 27.5
Equity Incentive Plan 2015
Shares Available For Grant
Additional shares authorized		23.0
RSUs and PSAs granted (in shares)		(18.5)
RSUs and PSAs canceled		5.5
Options canceled/expired (in shares)		1.0
Ending Balance, Number of Shares		33.5