Fair Value Measurements (Details) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
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	119,012,000	60,409,000
Payments to Acquire Marketable Securities	208,694,000	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0
Available-for-sale marketable securities:
Total fair value of available-for-sale marketable securities	1,463,006,000		$ 1,367,741,000
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
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Proceeds from Sale and Maturity of Marketable Securities	0	0
Payments to Acquire Marketable Securities	0	0
Marketable Securities, Unrealized Gain (Loss)	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	54,650,000		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	1,408,356,000		1,318,522,000
Unrestricted [Member] | Measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Available-for-sale marketable securities:
U.S. government and government agency obligations	0		0
Corporate bonds	0		0
Municipal bonds	0		0
Commercial Paper	0		0
Certificates of Deposit, at Carrying Value	0		0
Equity securities	54,650,000		49,219,000
Unrestricted [Member] | Measured on a recurring basis | Significant Other Observable Inputs (Level 2)
Available-for-sale marketable securities:
U.S. government and government agency obligations	125,492,000		132,340,000
Corporate bonds	1,205,974,000		1,072,851,000
Municipal bonds	4,625,000		7,644,000
Commercial Paper	32,620,000		63,075,000
Certificates of Deposit, at Carrying Value	39,645,000		42,612,000
Equity securities	0		0
Estimate of Fair Value Measurement [Member] | Measured on a recurring basis
Available-for-sale marketable securities:
Total fair value of available-for-sale marketable securities	1,463,006,000		1,367,741,000
Estimate of Fair Value Measurement [Member] | Unrestricted [Member] | Measured on a recurring basis
Available-for-sale marketable securities:
U.S. government and government agency obligations	125,492,000		132,340,000
Corporate bonds	1,205,974,000		1,072,851,000
Municipal bonds	4,625,000		7,644,000
Commercial Paper	32,620,000		63,075,000
Certificates of Deposit, at Carrying Value	39,645,000		42,612,000
Equity securities	$ 54,650,000		$ 49,219,000