Fair Value Measurements (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 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	$ 0	$ 0
Proceeds from Sale and Maturity of Marketable Securities			272,166	102,177
Payments to Acquire Marketable Securities			477,408	228,942
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0	0	0
Available-for-sale marketable securities:
Total fair value of available-for-sale marketable securities	1,588,214		1,588,214		$ 1,367,741
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	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	0	0
Payments to Acquire Marketable Securities	0	0	0	0
Marketable Securities, Unrealized Gain (Loss)	0	$ 0	0	$ 0
Measured on a recurring basis | Fair Value, Inputs, Level 1 [Member]
Available-for-sale marketable securities:
Total fair value of available-for-sale marketable securities	60,369		60,369		49,219
Measured on a recurring basis | Fair Value, Inputs, Level 2 [Member]
Available-for-sale marketable securities:
Total fair value of available-for-sale marketable securities	1,527,845		1,527,845		1,318,522
Measured on a recurring basis | Unrestricted [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale marketable securities:
Corporate bonds	0		0		0
Financial Instruments, Owned, US Government and Agency Obligations, at Fair Value	0		0		0
Municipal bonds	0		0		0
Commercial Paper	0		0		0
Certificates of Deposit, at Carrying Value	0		0		0
Equity securities	60,369		60,369		49,219
Measured on a recurring basis | Unrestricted [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale marketable securities:
Corporate bonds	1,349,987		1,349,987		1,072,851
Financial Instruments, Owned, US Government and Agency Obligations, at Fair Value	147,063		147,063		132,340
Municipal bonds	4,616		4,616		7,644
Commercial Paper	13,614		13,614		63,075
Certificates of Deposit, at Carrying Value	12,565		12,565		42,612
Equity securities	0		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	1,588,214		1,588,214		1,367,741
Estimate of Fair Value Measurement [Member] | Measured on a recurring basis | Unrestricted [Member]
Available-for-sale marketable securities:
Corporate bonds	1,349,987		1,349,987		1,072,851
Financial Instruments, Owned, US Government and Agency Obligations, at Fair Value	147,063		147,063		132,340
Municipal bonds	4,616		4,616		7,644
Commercial Paper	13,614		13,614		63,075
Certificates of Deposit, at Carrying Value	12,565		12,565		42,612
Equity securities	$ 60,369		$ 60,369		$ 49,219