Share-Based Compensation Activity Other Than Stock Option (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Service Award Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted during the period	4,583		4,583	18,434
Weighted average grant-date fair value per share granted during the period	$ 35.46		$ 35.46	$ 42.72
Fair value of shares vested during the period	$ 1,292	$ 1,531	$ 1,740	$ 2,214
Performance Award Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted during the period			34,229	22,511
Weighted average grant-date fair value per share granted during the period			$ 33.87	$ 44.44
Fair value of shares vested during the period	325	415	847	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	21,179	21,061	22,568	22,481
Weighted average grant-date fair value per share granted during the period	$ 35.41	$ 40.67	$ 35.49	$ 40.60
Fair value of shares vested during the period	554	57	660	173
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