Share-based Compensation - Summary of RSU's Granted Under LTIP (Detail) (2015 Long-Term Incentive Program [Member], USD $)	3 Months Ended
	Mar. 31, 2015
Awards with Market Conditions [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Units	32,414
Fair Value Per Unit at Grant Date	$ 30.80
Awards with Performance Conditions [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Units	44,885
Fair Value Per Unit at Grant Date	$ 22.24
Awards with Service Conditions [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Units	153,568
Fair Value Per Unit at Grant Date	$ 22.24