Income Taxes (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income before provision for income taxes [Abstract]
United States		$ 40,024,000	$ 15,362,000	$ 25,215,000
Foreign		134,000	0	0
Income before income taxes		40,158,000	15,362,000	25,215,000
Federal:
Current		14,605,000	5,800,000	5,773,000
Deferred		(316,000)	(3,095,000)	2,247,000
Total federal income tax expense		14,289,000	2,705,000	8,020,000
State:
Current		2,383,000	1,280,000	294,000
Deferred		455,000	(2,671,000)	(226,000)
Total state income tax expense		2,838,000	(1,391,000)	68,000
Foreign:
Current		118,000	0	0
Deferred		(85,000)	0	0
Total foreign income tax expense		33,000	0	0
Total income tax expense (benefit):
Current		17,106,000	7,080,000	6,067,000
Deferred		54,000	(5,766,000)	2,021,000
Total income tax expense (benefit)		17,160,000	1,314,000	8,088,000
Reconciliation of actual income tax rate and statutory federal income tax rate [Abstract]
Income tax expense at statutory rate (in hundredths)		35.00%	34.00%	34.00%
State income taxes (in hundredths)		0.90%	(7.70%)	1.70%
Stock-based compensation (in hundredths)		1.10%	(13.50%)	(0.40%)
R&D tax credit (in hundredths)		0.00%	(7.20%)	(2.60%)
Acquisition costs (in hundredths)		0.00%	3.00%	0.00%
Valuation allowance (in hundredths)		5.50%	0.00%	0.00%
Other (in hundredths)		0.20%	0.00%	(0.60%)
Effective income tax rate (in hundredths)		42.70%	8.60%	32.10%
Expected income tax benefit, federal research tax credit	1,300,000
Deferred tax assets:
Net operating loss carryforwards		2,917,000	1,979,000
Reserves and other tax benefits		22,527,000	18,457,000
Tax credits		2,415,000	3,176,000
Other		13,000	15,000
Deferred tax assets		27,872,000	23,627,000
Valuation allowance		(2,203,000)	0
Net deferred tax assets		25,669,000	23,627,000
Deferred tax liabilities:
Depreciation and amortization		(41,400,000)	(33,192,000)
Net deferred tax assets / (liabilities)		(15,731,000)	(9,565,000)
Reconciliation of unrecognized tax benefits [Roll Forward]
Balance of unrecognized tax benefits at January 1	5,445,000	4,364,000	2,955,000	2,179,000
Additions for tax positions of prior years		137,000	0	132,000
Additions for tax positions related to current year		1,009,000	1,409,000	644,000
Reductions for tax positions of prior years		(65,000)	0	0
Balance of unrecognized tax benefits at December 31		5,445,000	4,364,000	2,955,000
Unrecognized tax benefit decrease in effective tax rate		2,800,000
Federal [Member]
Reconciliation of actual income tax rate and statutory federal income tax rate [Abstract]
Net operating loss carryforwards		5,500,000
Operating loss carryforwards, expiration date		2028
Federal [Member] | Research and Development [Member]
Reconciliation of actual income tax rate and statutory federal income tax rate [Abstract]
Research and development tax credit carryforwards		4,100,000
Additional paid-in capital when realized associated with windfall tax benefits		4,100,000
Research and development tax credits carryforward period (in years)		20 years
Other Jurisdictions [Member]
Reconciliation of actual income tax rate and statutory federal income tax rate [Abstract]
Net operating loss carryforwards		900,000
Operating loss carryforwards, expiration date		2031
State [Member] | Research and Development [Member]
Reconciliation of actual income tax rate and statutory federal income tax rate [Abstract]
Research and development tax credit carryforwards		5,700,000
Additional paid-in capital when realized associated with windfall tax benefits		2,300,000
California [Member]
Reconciliation of actual income tax rate and statutory federal income tax rate [Abstract]
Net operating loss carryforwards		45,700,000
Net operating loss carryforwards associated with windfall tax benefits		$ 29,200,000
Operating loss carryforwards, expiration date		2016
Arizona [Member] | Research and Development [Member]
Reconciliation of actual income tax rate and statutory federal income tax rate [Abstract]
Research and development tax credits carryforward period (in years)		15 years