Fair Value Measurements Debt at at Carrying and Fair Value (Details) - USD ($) $ in Thousands		Mar. 31, 2020	Dec. 31, 2019
Liabilities:
Total long-term debt		$ 5,930,818	$ 5,920,325
Carrying Value [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		51,500
Subsidiaries [Member]
Liabilities:
Total long-term debt		4,336,612	4,336,142
Discounted cash flow [Member] | Carrying Value [Member]
Liabilities:
Total long-term debt		5,930,818	5,920,325
Discounted cash flow [Member] | Carrying Value [Member] | Level 2 [Member]
Liabilities:
Long-term debt (fixed-rate), net of discount	[1]	5,515,318	5,512,225
Long-term debt (variable-rate)		415,500	408,100
Debt issuance costs		23,300	24,100
Discounted cash flow [Member] | Fair Value [Member]
Liabilities:
Total long-term debt		7,345,719	7,412,416
Discounted cash flow [Member] | Fair Value [Member] | Level 2 [Member]
Liabilities:
Long-term debt (fixed-rate), net of discount	[1]	6,930,219	7,004,316
Long-term debt (variable-rate)		415,500	408,100
Discounted cash flow [Member] | Subsidiaries [Member] | Carrying Value [Member]
Liabilities:
Total long-term debt		4,336,612	4,336,142
Discounted cash flow [Member] | Subsidiaries [Member] | Carrying Value [Member] | Level 2 [Member]
Liabilities:
Long-term debt (fixed-rate), net of discount	[2]	4,336,612	4,336,142
Debt issuance costs		24,000	24,400
Discounted cash flow [Member] | Subsidiaries [Member] | Fair Value [Member]
Liabilities:
Total long-term debt		5,563,257	5,571,818
Discounted cash flow [Member] | Subsidiaries [Member] | Fair Value [Member] | Level 2 [Member]
Liabilities:
Long-term debt (fixed-rate), net of discount	[2]	$ 5,563,257	$ 5,571,818

[1] ¹ The carrying value includes debt issuances costs of $23.3 million and $24.1 million for March 31, 2020 and December 31, 2019, respectively, which are not included in fair value.

[2] ² The carrying value includes debt issuances costs of $24.0 million and $24.4 million for March 31, 2020 and December 31, 2019, respectively, which are not included in fair value.