Stock-based Compensation (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Stock-based Compensation
Number of shares authorized	8,193,088
Number of kinds of RSU's	2
Stock-based compensation expense	$ 6,800,000	$ 8,200,000
Stock Options
Stock-based Compensation
Term of award	10 years
Vesting period	3 years
Number of shares granted	3,375,908
Stock-based compensation expense	2,900,000	2,800,000
Stock options, shares
Balance at the beginning of the period (in shares)	3,358,408
Balance at the end of the period (in shares)	3,358,408		3,358,408
Vested and exercisable at the end of the period (in shares)	2,238,265
Vested and exercisable at the end of the period and expected to vest thereafter (in shares)	3,353,423
Stock options, exercise price
Balance at the beginning of the period (in dollars per share)	$ 20.39
Balance at the end of the period (in dollars per share)	$ 20.39		$ 20.39
Vested and exercisable at the end of the period (in dollars per share)	$ 20.20
Vested and exercisable at the end of the period and expected to vest thereafter (in dollars per share)	$ 20.39
Stock options, remaining contractual term
Beginning of the period	7 years 3 months		7 years 5 months 26 days
End of the period	7 years 3 months		7 years 5 months 26 days
Vested and exercisable at the end of the period	7 years 2 months 23 days
Vested and exercisable at the end of the period and expected to vest thereafter	7 years 3 months
Stock options, aggregate intrinsic value
Balance at the beginning of the period	4,813,000
Balance at the end of the period	29,980,000		4,813,000
Vested and exercisable at the end of the period	20,421,000
Vested and exercisable at the end of the period and expected to vest thereafter	29,934,000
RSUs
Stock-based Compensation
Number of shares granted	4,005,381
Stock-based compensation expense	$ 3,800,000	$ 5,400,000
Stock options, shares
Granted (in shares)	139,320
Book-value RSUs
Stock-based Compensation
Vesting period	3 years
Book-value RSUs | Minimum
Stock-based Compensation
Vesting period	3 years
Book-value RSUs | Maximum
Stock-based Compensation
Vesting period	4 years
TSR RSUs
Stock-based Compensation
Number of shares granted	263,569
Stock options, shares
Granted (in shares)	69,655
TSR RSUs | Minimum
Stock-based Compensation
Percentage of shares vested	0.00%
TSR RSUs | Maximum
Stock-based Compensation
Percentage of shares vested	200.00%