Investments (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended		9 Months Ended
	Aug. 05, 2015	Aug. 31, 2015	Mar. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Summary of Investments
Available-for-sale securities, Amortized Cost				$ 275,470	$ 24,914
Available-for-sale securities, Gross Unrealized Gains				164	13
Available-for-sale securities, Gross Unrealized Losses				(291)	(37)
Available-for-sale securities, Estimated Fair Value				275,343	24,890
Equity [Abstract]
Increase in total investments				250,500
Proceeds from the issuance of common stock, net	$ 218,600	$ 218,600	$ 188,800	407,477
Common Stock
Equity [Abstract]
Sale price of common stock (in dollars per share)	$ 63.75
Corporate debt securities
Summary of Investments
Available-for-sale securities due in one year or less, Amortized Cost				26,244	11,656
Available-for-sale securities due in one to three years, Amortized Cost				147,615	13,258
Available-for-sale securities due in one year or less, Gross Unrealized Gains				8	3
Available-for-sale securities due in one to three years, Gross Unrealized Gains				25	10
Available-for-sale securities due in one year or less, Gross Unrealized Losses				(10)	(4)
Available-for-sale securities due in one to three years, Gross Unrealized Losses				(276)	(33)
Available-for-sale securities due in one year or less, Estimated Fair Value				26,242	11,655
Available-for-sale securities due in one to three years, Estimated Fair Value				147,364	$ 13,235
Commercial Paper
Summary of Investments
Available-for-sale securities due in one year or less, Amortized Cost				7,488
Available-for-sale securities due in one year or less, Gross Unrealized Gains				2
Available-for-sale securities due in one year or less, Estimated Fair Value				7,490
Certificates of deposit
Summary of Investments
Available-for-sale securities due in one year or less, Amortized Cost				10,000
Available-for-sale securities due in one year or less, Gross Unrealized Gains				1
Available-for-sale securities due in one year or less, Estimated Fair Value				10,001
Municipal securities
Summary of Investments
Available-for-sale securities due in one year or less, Amortized Cost				13,874
Available-for-sale securities due in one to three years, Amortized Cost				70,249
Available-for-sale securities due in one year or less, Gross Unrealized Gains				11
Available-for-sale securities due in one to three years, Gross Unrealized Gains				117
Available-for-sale securities due in one to three years, Gross Unrealized Losses				(5)
Available-for-sale securities due in one year or less, Estimated Fair Value				13,885
Available-for-sale securities due in one to three years, Estimated Fair Value				$ 70,361