Share Based Compensation (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Authorized number of shares or units (in shares or units)	2,362,500
Number of share granted	2,691,225
Remaining shares available for grant (in shares)	0
Expiration term of award (in years)	10
Share-based compensation expense recognized [Abstract]
Stock compensation	$ 1,225,000	$ 972,000
Related deferred income tax benefit	(391,000)	(387,000)
Decrease in net income	590,000	585,000
Decrease in basic earnings per share (in dollars per share)	$ 0.02	$ 0.02
Decrease in diluted earnings per share (in dollars per share)	$ 0.02	$ 0.02
Weighted average exercise price [Roll Forward]
Exercisable, end of year (in dollars per share)	$ 15.89
General and Administrative Expense [Member]
Share-based compensation expense recognized [Abstract]
Stock compensation	981,000	972,000
Officers and Employees [Member]
Share-based compensation expense recognized [Abstract]
Stock compensation	1,000,000	1,000,000
Restricted Stock Units (Units) [Member]
Units activity [Roll Forward]
Outstanding at beginning of period (in units)	362,727
Granted (in units)	91,504
Forfeited (in units)	(12,384)
Vested (in units)	(80,483)
Non-vested Units at end of period (in units)	361,364
Weighted average grant fair value [Roll Forward]
Outstanding at beginning of period (in dollars per unit)	$ 31.43
Granted (in dollars per unit)	$ 31.10
Forfeited (in dollars per unit)	$ 31.71
Vested (in dollars per unit)	$ 30.71
Non-vested Units at end of period (in dollars per unit)	$ 31.49
Stock Options [Member]
Share-based compensation expense recognized [Abstract]
Aggregate intrinsic value of stock options exercised	129,000	3,800,000
Stock option activity [Roll Forward]
Outstanding, at beginning of period (in shares)	259,150
Options exercised (in shares)	(6,875)
Outstanding, at end of period (in shares)	252,275
Exercisable, end of year (in shares)	252,275
Weighted average exercise price [Roll Forward]
Outstanding, at beginning of period (in dollars per share)	$ 15.80
Options exercised (in dollars per share)	$ 12.17
Outstanding, at end of period (in dollars per share)	$ 15.89
Stock Incentive Plan 1994 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares cancelled	397,811
Share based compensation Stock Incentive Plan 2003 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of share granted	1,875,000
Vesting period	5 years
Shares cancelled	134,793
Share based compensation Stock Incentive Plan 2003 [Member] | Restricted Stock Units (Units) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of share granted	961,369
Share-based compensation expense recognized [Abstract]
Unrecognized compensation cost, restricted stock units	$ 9,700,000
Recognition period of unrecognized compensation cost (in years)	3 years 4 months
Share based compensation Stock Incentive Plan 2003 [Member] | Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of share granted	475,125