Fair Value Measurements (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale marketable securities:
Total fair value of available-for-sale marketable securities	$ 2,083,341,000	$ 1,367,741,000
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount		44,100,000
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	0	0
Proceeds from Sale and Maturity of Marketable Securities	544,584,000	274,456,000	$ 327,437,000
Payments to Acquire Marketable Securities	1,277,140,000	809,419,000	557,105,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other than Temporary Impairment Losses, Investments	0	0	$ 0
Significant Unobservable Inputs (Level 3)
Available-for-sale marketable securities:
Marketable Securities, Unrealized Gain (Loss)	0	0
Proceeds from Sale and Maturity of Marketable Securities	0	0
Payments to Acquire Marketable Securities	0	0
Measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Available-for-sale marketable securities:
Total fair value of available-for-sale marketable securities	62,785,000	49,219,000
Measured on a recurring basis | Significant Other Observable Inputs (Level 2)
Available-for-sale marketable securities:
Total fair value of available-for-sale marketable securities	2,020,556,000	1,318,522,000
Measured on a recurring basis | Unrestricted | Quoted Prices in Active Markets for Identical Assets (Level 1)
Available-for-sale marketable securities:
Financial Instruments, Owned, Corporate Debt, at Fair Value	0	0
Financial Instruments, Owned, US Government and Agency Obligations, at Fair Value	0	0
Municipal Debt Securities, at Carrying Value	0	0
Commercial Paper	0	0
Certificates of Deposit, at Carrying Value	0	0
Available-for-sale Securities, Equity Securities	62,785,000	49,219,000
Measured on a recurring basis | Unrestricted | Significant Other Observable Inputs (Level 2)
Available-for-sale marketable securities:
Financial Instruments, Owned, Corporate Debt, at Fair Value	1,712,480,000	1,072,851,000
Financial Instruments, Owned, US Government and Agency Obligations, at Fair Value	185,492,000	132,340,000
Municipal Debt Securities, at Carrying Value	4,587,000	7,644,000
Commercial Paper	106,973,000	63,075,000
Certificates of Deposit, at Carrying Value	11,024,000	42,612,000
Available-for-sale Securities, Equity Securities	0	0
Fair Value | Measured on a recurring basis
Available-for-sale marketable securities:
Total fair value of available-for-sale marketable securities	2,083,341,000	1,367,741,000
Fair Value | Measured on a recurring basis | Unrestricted
Available-for-sale marketable securities:
Financial Instruments, Owned, Corporate Debt, at Fair Value	1,712,480,000	1,072,851,000
Financial Instruments, Owned, US Government and Agency Obligations, at Fair Value	185,492,000	132,340,000
Municipal Debt Securities, at Carrying Value	4,587,000	7,644,000
Commercial Paper	106,973,000	63,075,000
Certificates of Deposit, at Carrying Value	11,024,000	42,612,000
Available-for-sale Securities, Equity Securities	$ 62,785,000	$ 49,219,000