Contractual interest rate for funds borrowed, under the debt agreement.

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Percentage price of original principal amount of debt at which debt can be redeemed by the issuer.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Long-Term Debt - Senior Unsecured Notes (Details) - USD ($) $ in Thousands	1 Months Ended			12 Months Ended
	Oct. 31, 2017	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Long Term Debt
Outstanding Balance				$ 2,376,723	$ 2,381,940
Senior Notes \| Maximum
Long Term Debt
Maximum redemption of debt instrument (as a percent)				35.00%
5.125% senior notes due 2021
Long Term Debt
Outstanding Balance				$ 700,000
Stated interest rate				5.125%
5.125% senior notes due 2021 \| Debt Instrument, Redemption, Period Four [Member]
Long Term Debt
Redemption price of debt instrument (as a percent)				100.00%
6 3/8% Senior Notes, due 2022
Long Term Debt
Stated interest rate				6.375%
Redemption price of debt instrument (as a percent)	103.188%	103.563%
5 1/4% Senior Notes due 2023
Long Term Debt
Outstanding Balance				$ 400,000
Stated interest rate				5.25%
5 1/4% Senior Notes due 2023 \| Debt Instrument, Redemption, Period Three [Member]
Long Term Debt
Redemption price of debt instrument (as a percent)				101.75%
5 1/4% Senior Notes due 2023 \| Debt Instrument, Redemption, Period Four [Member]
Long Term Debt
Redemption price of debt instrument (as a percent)				100.875%
5 1/4% Senior Notes due 2023 \| Debt Instrument, Redemption, Period Five [Member]
Long Term Debt
Redemption price of debt instrument (as a percent)				100.00%
5.500% senior notes due 2024
Long Term Debt
Outstanding Balance				$ 500,000
Stated interest rate				5.50%
5.500% senior notes due 2024 \| Debt Instrument, Redemption, Period Two [Member]
Long Term Debt
Redemption price of debt instrument (as a percent)				102.75%
5.500% senior notes due 2024 \| Debt Instrument, Redemption, Period Three [Member]
Long Term Debt
Redemption price of debt instrument (as a percent)				101.833%
5.500% senior notes due 2024 \| Debt Instrument, Redemption, Period Four [Member]
Long Term Debt
Redemption price of debt instrument (as a percent)				100.917%
5.500% senior notes due 2024 \| Debt Instrument, Redemption, Period Five [Member]
Long Term Debt
Redemption price of debt instrument (as a percent)				100.00%
4.125% Senior Note due 2025 [Member]
Long Term Debt
Outstanding Balance				$ 350,000
Stated interest rate		4.125%		4.125%
4.125% Senior Note due 2025 [Member] \| Debt Instrument, Redemption, Period One [Member]
Long Term Debt
Redemption price of debt instrument (as a percent)				104.125%
4.125% Senior Note due 2025 [Member] \| Debt Instrument, Redemption, Period Two [Member]
Long Term Debt
Redemption price of debt instrument (as a percent)				102.063%
4.125% Senior Note due 2025 [Member] \| Debt Instrument, Redemption, Period Three [Member]
Long Term Debt
Redemption price of debt instrument (as a percent)				101.031%
4.125% Senior Note due 2025 [Member] \| Debt Instrument, Redemption, Period Four [Member]
Long Term Debt
Redemption price of debt instrument (as a percent)				100.00%
Senior Notes 5.000 Percent Due 2026 [Member]
Long Term Debt
Outstanding Balance				$ 400,000
Stated interest rate				5.00%
Senior Notes 5.000 Percent Due 2026 [Member] \| Debt Instrument, Redemption, Period One [Member]
Long Term Debt
Redemption price of debt instrument (as a percent)				105.00%
Senior Notes 5.000 Percent Due 2026 [Member] \| Debt Instrument, Redemption, Period Two [Member]
Long Term Debt
Redemption price of debt instrument (as a percent)				102.50%
Senior Notes 5.000 Percent Due 2026 [Member] \| Debt Instrument, Redemption, Period Three [Member]
Long Term Debt
Redemption price of debt instrument (as a percent)				101.667%
Senior Notes 5.000 Percent Due 2026 [Member] \| Debt Instrument, Redemption, Period Four [Member]
Long Term Debt
Redemption price of debt instrument (as a percent)				100.833%
Senior Notes 5.000 Percent Due 2026 [Member] \| Debt Instrument, Redemption, Period Five [Member]
Long Term Debt
Redemption price of debt instrument (as a percent)				100.00%
6 1/8% senior notes, due 2019
Long Term Debt
Stated interest rate				6.125%
Redemption price of debt instrument (as a percent)			103.388%