Investments - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Money market funds	$ 39,600		$ 39,600
Fair Value	632,356		632,356		$ 636,878
Debt securities	620,374		620,374
Realized losses from the sale of available-for-sale securities (less than in 2015)	100	$ 400	100	$ 800
Sales of investments	11,800	5,600	140,107	8,239
Maturities of investments	185,000	$ 35,000	310,000	$ 60,000
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Fair Value	256,839		256,839
US Government Agencies Debt Securities
Schedule of Available-for-sale Securities [Line Items]
US Government Securities, at Carrying Value	259,600		259,600
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	103,913		103,913		$ 100,105
Debt securities	103,900		103,900
Other Debt Obligations
Schedule of Available-for-sale Securities [Line Items]
Other Marketable Securities, Noncurrent	$ 12,000		$ 12,000
Required percent of investment account held in cash or cash equivalents | Minimum
Schedule of Available-for-sale Securities [Line Items]
Investments, target portfolio allocations			50.00%
Other aggregated investments | Minimum | Securities of any one issuer (excluding the U.S. Government)
Schedule of Available-for-sale Securities [Line Items]
Investments, portfolio allocations requiring additional consent			10.00%
Other aggregated investments | Maximum | Securities of any one issuer (excluding the U.S. Government)
Schedule of Available-for-sale Securities [Line Items]
Investments, target portfolio allocations			15.00%
Investments, portfolio allocations requiring additional consent			15.00%
Investor | Fairholme Capital Management, L.L.C.
Schedule of Available-for-sale Securities [Line Items]
Common stock ownership percentage	27.10%		27.10%