Share-Based Compensation (Detail 3) - USD ($) $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Summary of changes in restricted stock and related information
Weighted-Average Grant Date Fair Value Granted (in dollars per share)	$ 90.47
Restricted stock
Summary of changes in restricted stock and related information
Unvested at the beginning of the period (in shares)	525,000
Granted (in shares)	100,000
Vested (in shares)	(145,663)	(180,799)
Forfeited (in shares)	(5,000)
Unvested at the end of the period (in shares)	474,000
Weighted-Average Grant Date Fair Value, beginning of the period (in dollars per share)	$ 60.60
Weighted-Average Grant Date Fair Value Granted (in dollars per share)	90.47	$ 73.64
Weighted-Average Grant Date Fair Value Vested (in dollars per share)	54.80
Weighted-Average Grant Date Fair Value Forfeited (in dollars per share)	68.00
Weighted-Average Grant Date Fair Value, end of the period (in dollars per share)	$ 68.60
Total fair value of awards vested during the period	$ 8.0	$ 8.0
Unrecognized compensation cost	$ 16.2
Period for recognition of unrecognized compensation cost	3 years 3 months 18 days
Cash-settled restricted stock units
Summary of changes in restricted stock and related information
Unvested at the beginning of the period (in shares)	174,000
Granted (in shares)	14,000
Vested (in shares)	(44,000)
Forfeited (in shares)	(3,000)
Unvested at the end of the period (in shares)	141,000