Share-Based Compensation Activity Other Than Stock Option (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Service Award Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted during the period	45,402		49,985	18,434
Weighted average grant-date fair value per share granted during the period	$ 36.10		$ 36.04	$ 42.72
Fair value of shares vested during the period	$ 266	$ 322	$ 2,006	$ 2,536
Performance Award Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted during the period		29,992	34,229	52,503
Weighted average grant-date fair value per share granted during the period		$ 41.52	$ 33.87	$ 42.77
Fair value of shares vested during the period	236		1,083	804
Board DSUs And Dividend Equivalents Granted To Board
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted during the period	1,298	6,232	23,866	28,798
Weighted average grant-date fair value per share granted during the period	$ 38.06	$ 38.78	$ 35.63	$ 40.19
Fair value of shares vested during the period	158	113	818	624
Chief Executive Officer | DSU Service Award
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted during the period			1,181	900
Weighted average grant-date fair value per share granted during the period			$ 33.87	$ 44.44
Fair value of shares vested during the period			568	704
Chief Executive Officer | Dsu Performance Award
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted during the period			59,049	24,122
Weighted average grant-date fair value per share granted during the period			$ 33.87	$ 44.44
Fair value of shares vested during the period			$ 396	$ 227