Stock-Based Compensation - RSU activity (Details) - Management Stock Purchase Plan - Restricted stock units (RSUs) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
RSU activity and related information
Outstanding at beginning of period (in shares)	110	154	174
Granted (in shares)	28	37	36
Cancelled/Forfeitures (in shares)	(40)	(79)	(46)
Settled (in shares)	(3)	(2)	(10)
Outstanding at end of period (in shares)	95	110	154
Vested at end of period (in shares)	32	35	66
Weighted Average Purchase Price
Outstanding at beginning of period (in dollars per share)	$ 57.91	$ 45.02	$ 39.68
Granted (in dollars per share)	69.76	63.77	61.84
Cancelled/Forfeitures (in dollars per share)	49.76	35.63	37.34
Settled (in dollars per share)	65.69	56.25	48.82
Outstanding at end of period (in dollars per share)	64.54	57.91	45.02
Vested at end of period (in dollars per share)	61.89	$ 52.67	$ 38.17
Weighted Average Intrinsic Value
Outstanding at end of period (in dollars per share)	57.16
Vested at end of period (in dollars per share)	$ 59.81
Aggregate intrinsic value of outstanding awards	$ 5.4
Aggregate intrinsic value of awards vested	1.9
Total intrinsic value of awards settled	$ 2.3	$ 3.5	$ 1.8