Fair Value Measurements Debt at at Carrying and Fair Value (Details) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Liabilities:
Total long-term debt		$ 5,645,191	$ 5,257,929
Subsidiaries [Member]
Liabilities:
Total long-term debt		3,895,042	3,549,911
Discounted cash flow [Member] | Subsidiaries [Member] | Carrying Value [Member]
Liabilities:
Total long-term debt		3,895,042	3,749,911
Discounted cash flow [Member] | Subsidiaries [Member] | Carrying Value [Member] | Level 2 [Member]
Liabilities:
Junior subordinated notes		0	250,000
Long-term debt (fixed-rate), net of discount	[1]	3,895,042	3,499,911
Debt issuance costs	[1]	24,200	24,600
Discounted cash flow [Member] | Subsidiaries [Member] | Fair Value [Member]
Liabilities:
Total long-term debt		4,598,167	4,789,065
Discounted cash flow [Member] | Subsidiaries [Member] | Fair Value [Member] | Level 2 [Member]
Liabilities:
Junior subordinated notes		0	238,935
Long-term debt (fixed-rate), net of discount	[1]	$ 4,598,167	$ 4,550,130

[1] 1 The carrying value includes debt issuances costs of $26.6 million and $27.9 million for June 30, 2018 and December 31, 2017, respectively, which are not included in fair value.