Share-Based Compensation Expense (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based compensation expense
Pre-tax compensation expense	$ 6.8	$ 5.1
Income tax benefit	(2.5)	(1.9)
Total share-based compensation expense, net of income taxes	4.3	3.2
Stock Option Awards
Share-based compensation expense
Pre-tax compensation expense	2.1	1.8
Income tax benefit	(0.8)	(0.7)
Total share-based compensation expense, net of income taxes	1.3	1.1
Restricted Stock Units (RSUs)
Share-based compensation expense
Pre-tax compensation expense	2.9	2.1
Income tax benefit	(1.1)	(0.8)
Total share-based compensation expense, net of income taxes	$ 1.8	1.3
Cancelled (in shares)	(11)
Vested (in shares)	113
Performance Shares
Share-based compensation expense
Performance share units awarded	44		47	58	45
Service period over which compensation expense would be amortized	3 years
Weighted average fair value of the shares granted	$ 131.34		$ 77.54	$ 52.03	$ 67.19
Cancelled (in shares)	0
Award payout achieved (as a percent)	200.00%
Vested (in shares)	90
Unrecognized compensation costs	$ 7.0
Remaining requisite service period (in years)	2 years 3 months 18 days
Performance Shares | Minimum
Share-based compensation expense
Performance shares available for vesting (as a percent)	0.00%
Performance Shares | Maximum
Share-based compensation expense
Performance shares available for vesting (as a percent)	200.00%
Performance Shares And Other Share Based Awards
Share-based compensation expense
Pre-tax compensation expense	$ 1.8	1.2
Income tax benefit	(0.6)	(0.4)
Total share-based compensation expense, net of income taxes	$ 1.2	$ 0.8