Stockholders' Equity and Share-Based Compensation (Details) (USD $)	1 Months Ended		3 Months Ended		6 Months Ended
	Aug. 31, 2012	May 31, 2012	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Stockholders' Equity and Share Based Compensation (Additional Textual) [Abstract]
Weighted average fair value for stock granted					$ 41.87	$ 39.02
Shares issued relating to vesting of restricted stock units and exercise of stock options			63,000	78,000	222,000	216,000
Recognized total share-based compensation costs			$ 2,300,000	$ 2,200,000	$ 4,236,000	$ 4,159,000
Total unrecognized share-based compensation cost which is expected to be recognized			24,400,000		24,400,000
Cash dividend declared per share	$ 0.175	$ 0.175	$ 0.175	$ 0.15	$ 0.35	$ 0.30
Dividend paid in cash					$ (11,696,000)	$ (10,314,000)
Restricted Stock [Member]
Stockholders' equity and share- based compensation (Textual) [Abstract]
Stock units granted					300,298	286,783
Performance-based share awards [Member]
Stockholders' equity and share- based compensation (Textual) [Abstract]
Stock units granted					32,834