Share-Based Compensation Activity Other Than Stock Option (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Performance Award Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted during the period		34,229
Weighted average grant-date fair value per share granted during the period		$ 33.87
Fair value of shares vested during the period	$ 1,086	$ 522
DSU Service Award
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted during the period		1,283
Weighted average grant-date fair value per share granted during the period		$ 34.03
Number of Performance DSUs converted to common shares of Company stock during period	47,189
Fair value of shares vested during the period	707	572
Dsu Performance Award
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted during the period		59,049
Weighted average grant-date fair value per share granted during the period		$ 33.87
Number of Performance DSUs converted to common shares of Company stock during period	44,078
Fair value of shares vested during the period	1,244	396
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	771	1,286
Weighted average grant-date fair value per share granted during the period	$ 46.98	$ 36.74
Fair value of shares vested during the period	107	102
Service Award Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares granted during the period	114,705
Weighted average grant-date fair value per share granted during the period	$ 46.79
Fair value of shares vested during the period	$ 392	$ 448