Convertible Senior Notes (Narrative) (Details)		1 Months Ended		3 Months Ended			12 Months Ended
	Apr. 30, 2014 $ / shares shares	Feb. 28, 2017	Jan. 31, 2017 $ / shares	Mar. 31, 2017 USD ($) shares	Mar. 31, 2016 USD ($)	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)	Feb. 01, 2017 $ / shares	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Amount used to repurchase stock				$ 500,000,000	$ 28,689,000
Convertible notes				0					$ 1,348,156,000
Temporary equity				0					79,495,000
Purchase From Accelerated Share Repurchase [Member]
Debt Instrument [Line Items]
Stock repurchased during period, value						$ 1,400,000,000
Senior Notes Due 2019 [Member]
Debt Instrument [Line Items]
Convertible debt				1,357,580,000			$ 1,440,000,000		1,348,156,000
Stated interest rate percentage							0.50%
Proceeds from convertible debt							$ 1,420,000,000
Payments for (proceeds from) hedge, investing activities							$ 82,600,000
Stock repurchased during period, value						1,500,000,000
Convertible debt, conversion ratio		0.0139061	0.011111
Debt instrument, face amount				$ 1,000
Convertible debt, conversion price (in dollars per share) | $ / shares			$ 90					$ 71.91
Repurchase price as a percent of principal amount				100.00%
Amortization of debt discount, effective interest method, percent				3.00%
Convertible notes				$ 1,360,000,000
Deferred tax liability, equity component				8,200,000
Temporary equity				$ 0					$ 79,495,000
Shares of common stock covered by note hedges (shares) | shares				16,000,000
Warrant transaction (shares) | shares	16,000,000
Adjustment to number of shares of common stock covered by note hedges and warrant transactions (shares) | shares				20,000,000
Strike price of warrants (in dollars per share) | $ / shares	$ 120.0							$ 95.25
Warrants exercised (shares) | shares				0
Privately Negotiated Transaction [Member]
Debt Instrument [Line Items]
Amount used to repurchase stock						$ 101,000,000