Fair Value Measurements (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	$ 0	$ 0
Proceeds from Sale and Maturity of Marketable Securities	$ 255,276	119,012
Document Period End Date	Mar. 31, 2018
Payments to Acquire Marketable Securities	$ 601,313	208,694
Marketable securities:
Securities Owned Not Readily Marketable	37,500
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other than Temporary Impairment Losses, Investments	0	$ 0
Measured on a recurring basis | Fair Value, Inputs, Level 1 [Member]
Marketable securities:
Marketable Securities	69,075		$ 62,785
Measured on a recurring basis | Fair Value, Inputs, Level 2 [Member]
Marketable securities:
Marketable Securities	2,358,371		2,020,556
Measured on a recurring basis | Unrestricted [Member] | Fair Value, Inputs, Level 1 [Member]
Marketable securities:
Corporate bonds	0		0
Financial Instruments, Owned, US Government and Agency Obligations, at Fair Value	0		0
Municipal bonds	0		0
Commercial Paper	0		0
Certificates of Deposit, at Carrying Value	0		0
Marketable Securities, Equity Securities	69,075		62,785
Measured on a recurring basis | Unrestricted [Member] | Fair Value, Inputs, Level 2 [Member]
Marketable securities:
Corporate bonds	2,110,609		1,712,480
Financial Instruments, Owned, US Government and Agency Obligations, at Fair Value	154,700		185,492
Municipal bonds	2,580		4,587
Commercial Paper	70,385		106,973
Certificates of Deposit, at Carrying Value	20,097		11,024
Marketable Securities, Equity Securities	0		0
Estimate of Fair Value Measurement [Member] | Measured on a recurring basis
Marketable securities:
Marketable Securities	2,427,446		2,083,341
Estimate of Fair Value Measurement [Member] | Measured on a recurring basis | Unrestricted [Member]
Marketable securities:
Corporate bonds	2,110,609		1,712,480
Financial Instruments, Owned, US Government and Agency Obligations, at Fair Value	154,700		185,492
Municipal bonds	2,580		4,587
Commercial Paper	70,385		106,973
Certificates of Deposit, at Carrying Value	20,097		11,024
Marketable Securities, Equity Securities	$ 69,075		$ 62,785