Incentive Plans - Summary of Activity Relating to PRSUs (Detail) (PRSUs [Member], USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
PRSUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Units, Beginning Balance	448,128
Units, Granted	360,783	448,128
Units, Ending Balance	808,911	448,128
Weighted Average Grant Date Fair Value, Beginning Balance	$ 36.11
Weighted Average Grant Date Fair Value, Granted	$ 44.46	$ 36.11
Weighted Average Grant Date Fair Value, Ending Balance	$ 39.83	$ 36.11