Investments (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	0 Months Ended	1 Months Ended	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Summary of Investments
Total investments	$ 117,294,000	$ 117,294,000	$ 117,294,000	$ 24,914,000
Available-for-sale securities, Gross Unrealized Gains	60,000	60,000	60,000	13,000
Available-for-sale securities, Gross Unrealized Losses	(16,000)	(16,000)	(16,000)	(37,000)
Available-for-sale securities, Estimated Fair Value	117,338,000	117,338,000	117,338,000	24,890,000
Underwriting Discounts For Initial Public Offering	12,100,000	12,100,000
Underwriting Commissions And Other Offering Expenses	200,000	200,000
Equity [Abstract]
Increase in total investments		92,400,000
Number of shares sold in an underwritten public offering		4.6
Sale price of common stock (in dollars per share)	$ 44.00	$ 44.00	$ 44.00
Proceeds from the issuance of common stock, net of underwriting discounts and other expenses		201,200,000	189,150,000
Corporate debt securities
Summary of Investments
Available-for-sale securities due in one year or less, Amortized Cost	15,167,000	15,167,000	15,167,000	11,656,000
Available-for-sale securities due in one to three years, Amortized Cost	47,352,000	47,352,000	47,352,000	13,258,000
Available-for-sale securities due in one year or less, Gross Unrealized Gains	13,000	13,000	13,000	3,000
Available-for-sale securities due in one to three years, Gross Unrealized Gains	45,000	45,000	45,000	10,000
Available-for-sale securities due in one year or less, Gross Unrealized Losses				(4,000)
Available-for-sale securities due in one to three years, Gross Unrealized Losses	(10,000)	(10,000)	(10,000)	(33,000)
Available-for-sale securities due in one year or less, Estimated Fair Value	15,180,000	15,180,000	15,180,000	11,655,000
Available-for-sale securities due in one to three years, Estimated Fair Value	47,387,000	47,387,000	47,387,000	13,235,000
Commercial Paper
Summary of Investments
Available-for-sale securities due in one year or less, Amortized Cost	30,460,000	30,460,000	30,460,000
Available-for-sale securities due in one year or less, Gross Unrealized Gains	2,000	2,000	2,000
Available-for-sale securities due in one year or less, Gross Unrealized Losses	(1,000)	(1,000)	(1,000)
Available-for-sale securities due in one year or less, Estimated Fair Value	30,461,000	30,461,000	30,461,000
Certificates of deposit
Summary of Investments
Available-for-sale securities due in one year or less, Amortized Cost	15,000,000	15,000,000	15,000,000
Available-for-sale securities due in one year or less, Gross Unrealized Losses	(1,000)	(1,000)	(1,000)
Available-for-sale securities due in one year or less, Estimated Fair Value	14,999,000	14,999,000	14,999,000
Municipal securities
Summary of Investments
Available-for-sale securities due in one to three years, Amortized Cost	9,315,000	9,315,000	9,315,000
Available-for-sale securities due in one to three years, Gross Unrealized Losses	(4,000)	(4,000)	(4,000)
Available-for-sale securities due in one to three years, Estimated Fair Value	$ 9,311,000	$ 9,311,000	$ 9,311,000