Employee Stock-Based Compensation and Benefit Plans (Narrative) (Details) $ / shares in Units, $ in Thousands		1 Months Ended				3 Months Ended	12 Months Ended			13 Months Ended	17 Months Ended
	Aug. 01, 2017	Mar. 31, 2017 $ / shares shares	Mar. 31, 2016 $ / shares shares	Jan. 31, 2016 $ / shares shares	Mar. 31, 2015 $ / shares shares	Mar. 31, 2017	Dec. 31, 2017 USD ($) installment plan $ / shares shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Mar. 31, 2015	May 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock-based compensation plans offered | plan							1
Stock-based compensation | $							$ 165,120	$ 152,739	$ 128,262
Incremental share-based compensation expense to be recognized | $							6,500
Deferred tax asset related to stock-based compensation | $							46,100	53,500	40,200
Tax benefit from compensation expense | $							$ 72,900	64,700	50,500
401(k) Benefit Plan [Abstract]
Maximum annual contribution per employee (as a percent)							90.00%
Contribution per dollar of employee contribution							50.00%
Matching percent							3.00%
Defined Contribution Plan, Cost | $							$ 13,700	14,000	12,100
Employer contributions, vesting period											4 years
Employer contributions, annual vesting rate											25.00%
Market and service condition stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-vested stock unit awards granted to senior level employees (shares) | shares		275,148	234,816		393,464
Performance period		3 years					3 years
Total return to shareholders as percentile of peer group, minimum		41.00%
Total return to shareholders as percentile of peer group, maximum		80.00%
Maximum share cap based on target shareholder return (percent)		100.00%
Minimum share cap based on target shareholder return (percent)		200.00%
Minimum peer group percentile to achieve target share cap		80.00%
Period to determine actual stock grant following end of interim measurement period		60 days
Period to determine actual stock grant following end of performance period							60 days
Maximum percentage of market and service condition stock units that will ultimately vest							200.00%
Minimum overperformance percentage against benchmark index return for maximum stock issuance, percentage							40.00%
Maximum underperformance against benchmark index return before company discontinues issuance of stock, percentage (more than 20%)							20.00%
Percent of award issued if return is negative but benchmark index is met							75.00%
Non-vested stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $							$ 149,800	135,700	117,900
Non-vested stock unit awards granted to senior level employees (shares) | shares							3,155,701
Number of shares represented by each award upon vesting (shares) | shares							1
Expected volatility rate, calculation basis, period		2 years 9 months				3 years				2 years 9 months 4 days
Granted (in dollars per share) | $ / shares		$ 104.05	$ 66.18	$ 49.68	$ 61.01		$ 86.41
Share-based compensation award, stock vesting period, monthly installments | installment							12
Fair value of awards released | $							$ 150,000	$ 163,800	132,900
Total unrecognized compensation cost related to stock-based compensation | $							$ 245,500
Total unrecognized compensation cost recognition period (in years)							1 year 9 months
Non-vested stock | Annual vesting on each anniversary
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage							33.33%
Performance stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance period			3 years
Maximum target shareholder return (percent)			10.00%
Maximum share cap based on target shareholder return (percent)			100.00%
Minimum target shareholder return (percent)			5.00%
Minimum share cap based on target shareholder return (percent)			200.00%
Target shareholder return maximum to attain 200% (percent)			30.00%
Period to determine actual stock grant following end of interim measurement period			60 days
Interim measurement period			18 months
Share cap based on interim measurement period (percent)			33.00%
Period to determine actual stock grant following end of performance period	60 days		60 days
Maximum percentage of market and service condition stock units that will ultimately vest	200.00%
Superseded Stock Plans | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award expiration period							5 years
Superseded Stock Plans | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award expiration period							10 years
2014 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance under the Plan (shares) | shares							46,000,000
Shares reserved for issuance under the Plans (shares) | shares							28,197,138
Shares available for grant under the 2014 Equity Incentive Plan (shares) | shares							23,521,681
2005 ESPP Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $									$ 6,500
2015 ESPP Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance under the Plan (shares) | shares							16,000,000
Employee Stock Purchase Plan, payment period							6 months
Employee Stock Purchase Plan, maximum number of shares per period that employees can purchase (shares) | shares							12,000
Employee Stock Purchase Plan, lower of purchase price offered on either first or last day of payment period as a percentage of fair market value (percent)							85.00%
Employee Stock Purchase Plan, employee disqualification, ownership percent of outstanding stock							5.00%
Employee Stock Purchase Plan, total shares issued under plan (shares) | shares							1,260,420
Expected volatility rate, calculation basis, period							6 months	6 months	6 months
2015 ESPP Plan | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee Stock Purchase Plan, option to purchase shares through payroll deduction, payroll deduction amount per pay period per employee, as a percentage of base pay							1.00%
2015 ESPP Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee Stock Purchase Plan, option to purchase shares through payroll deduction, payroll deduction amount per pay period per employee, as a percentage of base pay							10.00%
Employee Stock Purchase Plans
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation | $							$ 10,000	$ 7,500
Chief Executive Officer | Performance stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-vested stock unit awards granted to senior level employees (shares) | shares				220,235