Equity-based Incentive Plans (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Stock options
Equity-based Incentive Plans
Weighted-average fair value of options granted (in dollars per share)	$ 5.36	$ 5.81	$ 5.14
2006 Equity Incentive Plan (2006 Plan)
Other Share-based Compensation Disclosures
Period during which no other equity-based incentives were issued (in years)	P3Y
2006 Equity Incentive Plan (2006 Plan) | Stock options
Equity-based Incentive Plans
Original number of shares authorized for issuance	16,000,000
Remaining number of shares available for issuance	1,000,000
Options granted	2,742,904	2,545,932	1,637,107
Options vesting percentage	100.00%
Vesting period of options (in months)	6 months
Option term (in years)	P5Y
Options exercise price as a percentage of fair value of common stock	100.00%
Aggregate intrinsic value of options exercised (in dollars)	$ 7,400,000	$ 11,600,000	$ 26,200,000
Aggregate intrinsic value of options exercisable (in dollars)	12,100,000
Aggregate intrinsic value of options that are currently outstanding and that are expected to be exercised (in dollars)	13,100,000
Unrecognized stock option compensation expense	5,700,000
Fair value of stock option awards estimated on the date of grant using the Black Scholes option valuation model
Volatility minimum (as a percent)	65.10%	66.40%	67.20%
Volatility maximum (as a percent)	66.30%	67.00%	67.40%
Risk-free interest rate minimum (as a percent)	0.40%	0.80%	2.10%
Risk-free interest rate maximum (as a percent)	1.40%	2.00%	2.20%
Expected life minimum (in years)	2.8	2.8	2.2
Expected life maximum (in years)	4.1	4.1	3.9
2006 Equity Incentive Plan (2006 Plan) | Stock options | Minimum
Fair value of stock option awards estimated on the date of grant using the Black Scholes option valuation model
Dividend yield (as a percent)	2.30%	2.00%	1.70%
2006 Equity Incentive Plan (2006 Plan) | Stock options | Maximum
Fair value of stock option awards estimated on the date of grant using the Black Scholes option valuation model
Dividend yield (as a percent)	2.50%	2.20%	2.00%
2006 Equity Incentive Plan (2006 Plan) | Restricted stock
Equity-based Incentive Plans
Stock issued	249,000	43,300	157,000
2006 Equity Incentive Plan (2006 Plan) | Deferred stock units
Equity-based Incentive Plans
Stock issued	42,000
2008 Executive Incentive Compensation Plan (Executive Plan) | Stock options
Equity-based Incentive Plans
Original number of shares authorized for issuance	2,500,000
Remaining number of shares available for issuance	1,900,000
Vesting period of options (in months)	3 years
Other Share-based Compensation Disclosures
Market value of award	4,000,000	208,000
2004 Employee Stock Purchase Plan
Other Share-based Compensation Disclosures
Maximum annual allowable payroll deduction per employee	10,400
Employer's matching contribution of employees' payroll deductions (as a percent)	10.00%
Total expenses for the plan	$ 328,000	$ 299,000	$ 304,000