Fair Value Measurements - Debt at Carrying and Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Subsidiaries [Member] | Carrying Amount | Income Approach Valuation Technique
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Excluding Current Maturities, Fair Value Disclosure	$ 3,894,860	$ 3,749,911
Subsidiaries [Member] | Carrying Amount | Income Approach Valuation Technique | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unamortized Debt Issuance Expense	24,600	24,600
Junior subordinated notes	0	250,000
Long-term Debt, Fixed Rate, Net of Discount, Fair Value Disclosure	3,894,860	3,499,911
Subsidiaries [Member] | Total | Income Approach Valuation Technique
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Excluding Current Maturities, Fair Value Disclosure	4,574,611	4,789,065
Subsidiaries [Member] | Total | Income Approach Valuation Technique | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Junior subordinated notes	0	238,935
Long-term Debt, Fixed Rate, Net of Discount, Fair Value Disclosure	4,574,611	4,550,130
Parent Company [Member] | Carrying Amount | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Receivable, Fair Value Disclosure	49,500	48,500
Parent Company [Member] | Carrying Amount | Income Approach Valuation Technique
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Excluding Current Maturities, Fair Value Disclosure	5,672,491	5,457,929
Parent Company [Member] | Carrying Amount | Income Approach Valuation Technique | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unamortized Debt Issuance Expense	26,100	27,900
Junior subordinated notes	0	250,000
Long-term Debt, Fixed Rate, Net of Discount, Fair Value Disclosure	5,510,591	5,105,329
Long-term Debt, Variable Rate, Net of Discount, Fair Value Disclosure	161,900	102,600
Parent Company [Member] | Total | Income Approach Valuation Technique
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Excluding Current Maturities, Fair Value Disclosure	6,605,642	6,862,050
Parent Company [Member] | Total | Income Approach Valuation Technique | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Junior subordinated notes	0	238,935
Long-term Debt, Fixed Rate, Net of Discount, Fair Value Disclosure	6,443,742	6,520,515
Long-term Debt, Variable Rate, Net of Discount, Fair Value Disclosure	$ 161,900	$ 102,600