Summary of Restricted Stock Activity (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Time-Based Restricted Stock Units
Schedule of Share based Compensation Arrangements by Share based Payment Award, Equity Instruments, Other Than Options, Restricted Stock Units [Line Items]
Outstanding, beginning of period, Shares	750,837	768,529
Granted, Shares	124,850	168,750
Vested, Shares	(245,185)	(117,522)
Forfeited, Shares	(1,625)	(10,309)
Outstanding, end of period, Shares	628,877	809,448
Performance And Market-Based Restricted Stock Units
Schedule of Share based Compensation Arrangements by Share based Payment Award, Equity Instruments, Other Than Options, Restricted Stock Units [Line Items]
Outstanding, beginning of period, Shares	132,775	69,500
Granted, Shares	79,850	99,500
Outstanding, end of period, Shares	212,625	169,000
Weighted Average Grant Date Fair Value
Schedule of Share based Compensation Arrangements by Share based Payment Award, Equity Instruments, Other Than Options, Restricted Stock Units [Line Items]
Outstanding, beginning of period, Weighted Average Grant Date Fair Value	40.15	40.17
Granted, Weighted Average Grant Date Fair Value	42.49	36.93
Vested, Weighted Average Grant Date Fair Value	39.42	35.27
Forfeited, Weighted Average Grant Date Fair Value	39.32	39.67
Outstanding, end of period, Weighted Average Grant Date Fair Value	40.90	40.21
Performance-Based Weighted Average Grant Date Fair Value
Schedule of Share based Compensation Arrangements by Share based Payment Award, Equity Instruments, Other Than Options, Restricted Stock Units [Line Items]
Outstanding, beginning of period, Weighted Average Grant Date Fair Value	39.97	45.37
Granted, Weighted Average Grant Date Fair Value	41.67	36.61
Outstanding, end of period, Weighted Average Grant Date Fair Value	40.61	40.21