Short-term Investments - Assets Classified on Consolidated Balance Sheets (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Total, Estimated Fair Value	$ 2,962,630		$ 1,614,190
Fair Value, Measurements, Recurring [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, Estimated Fair Value	2,962,630		1,614,190
Cash and Cash Equivalents [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, Estimated Fair Value	2,454,777		1,113,608
Short-term Investments [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, Estimated Fair Value	502,931		494,888
Other Non-current Assets [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, Estimated Fair Value	$ 4,922	[1]	$ 5,694	[1]

[1] Primarily restricted cash that is related to workers compensation deposits.