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Percentage of voting interests to be acquired for business combination minimum.

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Share Based Compensation Arrangement by Share Based Payment Award Award Restricted Stock Units Vesting Period Minimum.

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Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to nondeductible expenses.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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Discount rate from fair value on purchase date that participants pay for shares.

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The highest percentage of annual salary that an employee is permitted to utilize with respect to the plan.

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Number of additional shares authorized for issuance under share-based payment arrangement.

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Number of shares authorized for issuance under share-based payment arrangement.

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Per share weighted-average price paid for shares purchased on open market for issuance under share-based payment arrangement.

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Intrinsic value of vested award under share-based payment arrangement. Excludes share and unit options.

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Number of shares issued during the period as a result of an employee stock purchase plan.

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Share-Based Compensation (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		12 Months Ended
	May 21, 2015	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2012	Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amount		$ 8.0	$ 5.0	$ 3.0
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount		$ 25.3
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition		2 years
Summary of restricted stock unit activity
Share-based Payment Arrangement, Expense		$ 31.0	$ 28.0	$ 29.0
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized	7.5
Period For Shares Issuance Following Departure Of Director		six months and one day
Period For Successive Overlapping		6 months
Percentage Of Voting Interests To Be Acquired For Business Combination Minimum		50.00%
Exercise Price Of Purchasing Rights As Percentage Of Fair Market Value In Case Of Acquisition		85.00%
Incentive Compensation Plan Two Thousand Eleven [Member]
Summary of restricted stock unit activity
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized					15.0
Restricted Stock Unit Activity Under 2011 Plan [Member]
Summary of restricted stock unit activity
Nonvested shares, beginning balance		1.7
Restricted stock unit activity granted, shares		1.3
Restricted stock unit activity granted, weighted average grant date fair value		$ 17.27	$ 20.62	$ 19.76
Restricted stock unit activity vested, shares		(0.9)
Restricted stock unit activity vested, weighted average grant date fair value		$ 20.29
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period		(0.1)
Restricted stock unit activity forfeited, weighted average grant date fair value		$ 18.99
Nonvested shares, ending balance		2.0	1.7
Nonvested, weighted average grant date fair value, ending balance		$ 18.59	$ 20.59
Crewmember Stock Purchase Plan 2011 [Member]
Summary of restricted stock unit activity
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized						8.0
Share-based Compensation Arrangement by Share-based Payment Award, Maximum Employee Subscription Rate		10.00%
Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Purchase Date		15.00%
Employee Stock Purchase Plan (ESPP), Expense		$ 9.0	$ 9.0	$ 8.0
Stock Issued During Period, Shares, Employee Stock Purchase Plans		3.2	3.2	2.5
Share-based Compensation Arrangement by Share-based Payment Award, Per Share Weighted Average Price of Shares Purchased		$ 16.06	$ 15.21	$ 17.46
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Vested		$ 15.0	$ 16.0	$ 20.0
Summary of restricted stock unit activity
Nonvested shares, ending balance		2.5
Restricted Stock Units (RSUs) [Member] \| Restricted Stock Unit Activity Under 2011 Plan [Member]
Summary of restricted stock unit activity
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period		3 years
Deferred Stock Units (DSU's) [Member]
Summary of restricted stock unit activity
Share Based Compensation Arrangement By Share Based Payment Award Award Restricted Stock Units Vesting Period Minimum		1 year
Share Based Compensation Arrangement By Share Based Payment Award Award Restricted Stock Units Vesting Period Maximum		3 years
Crewmember Stock Purchase Plan 2011 [Member]
Summary of restricted stock unit activity
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized	15.0