Stockholders' Equity (Details 2) (2005 Contingent stock plan [Member], USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Non-vested awards under the 2005 Contingent Stock Plan
Restricted stock shares issued for new awards	308,650	403,360	516,650
Restricted stock units awarded	(90,100)	(94,200)	(101,300)
Number of non-vested restricted stock Weighted- Average per Share Market Value on Grant Date, beginning of year	$ 21.46	$ 16.33
Number of non-vested restricted stock Weighted- Average per Share Market Value on Grant Date, Ending of year	$ 24.93	$ 21.46	$ 16.33
Restricted Stock [Member]
Non-vested awards under the 2005 Contingent Stock Plan
Number of shares available, beginning of year	1,188,346
Restricted stock shares issued for new awards	308,650	403,360	516,650
Restricted stock units awarded	(271,936)
Restricted stock shares forfeited	(41,400)	(19,133)	(38,431)
Number of shares available, end of year	1,183,660	1,188,346
Number of non-vested restricted stock Weighted- Average per Share Market Value on Grant Date, beginning of year	$ 19.78
Restricted stock for new awards Weighted- Average per Share Market Value on Grant Date during the year	$ 25.30
Restricted stock vested Weighted- Average per Share Market Value on Grant Date during the year	$ 23.45
Restricted stock forfeited Weighted- Average per Share Market Value on Grant Date during the year	$ 17.44
Number of non-vested restricted stock Weighted- Average per Share Market Value on Grant Date, Ending of year	$ 20.45	$ 19.78
Restricted Stock Units (RSUs) [Member]
Non-vested awards under the 2005 Contingent Stock Plan
Number of shares available, beginning of year	301,950
Restricted stock shares issued for new awards	133,650	115,150	106,200
Restricted stock units awarded	(90,100)
Restricted stock shares forfeited	(3,500)	(12,500)	(19,700)
Number of shares available, end of year	342,000	301,950
Number of non-vested restricted stock Weighted- Average per Share Market Value on Grant Date, beginning of year	$ 20.13
Restricted stock for new awards Weighted- Average per Share Market Value on Grant Date during the year	$ 24.40
Restricted stock vested Weighted- Average per Share Market Value on Grant Date during the year	$ 22.51
Restricted stock forfeited Weighted- Average per Share Market Value on Grant Date during the year	$ 20.37
Number of non-vested restricted stock Weighted- Average per Share Market Value on Grant Date, Ending of year	$ 21.17	$ 20.13
Cash Awards [Member]
Non-vested awards under the 2005 Contingent Stock Plan
Number of shares available, beginning of year	51,000
Restricted stock shares issued for new awards	30,550
Restricted stock units awarded	(22,200)
Restricted stock shares forfeited	(1,300)
Number of shares available, end of year	58,050