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Face (par) amount of debt instrument at time of issuance.

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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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Debt (Schedule of Long-term Debt Instruments) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Jun. 30, 2006
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 2,806,524,000	$ 2,808,770,000
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	18,651,000	20,405,000
Debt Instrument, Face Amount			$ 850,000,000.0
Long-term debt	2,787,873,000	2,788,365,000
Senior Notes Due 2017 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 300,000,000
Debt Instrument, Maturity Date	Dec. 31, 2017
Debt Instrument, Interest Rate, Stated Percentage	5.625%
Senior Notes Due 2019 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 399,941,000	399,873,000
Debt Instrument, Face Amount	$ 400,000,000
Debt Instrument, Maturity Date	Dec. 31, 2019
Debt Instrument, Interest Rate, Stated Percentage	6.45%
Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 399,466,000	399,245,000
Debt Instrument, Face Amount	$ 400,000,000
Debt Instrument, Maturity Date	Dec. 31, 2021
Debt Instrument, Interest Rate, Stated Percentage	5.00%
Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 399,288,000	399,138,000
Debt Instrument, Face Amount	$ 400,000,000
Debt Instrument, Maturity Date	Dec. 31, 2023
Debt Instrument, Interest Rate, Stated Percentage	4.70%
Senior Notes Due 2026 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 399,988,000	399,987,000
Debt Instrument, Face Amount	$ 400,000,000
Debt Instrument, Maturity Date	Dec. 31, 2026
Debt Instrument, Interest Rate, Stated Percentage	3.95%
Subordinated Debentures Due 2042 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 400,000,000	400,000,000
Debt Instrument, Face Amount	$ 400,000,000
Debt Instrument, Maturity Date	Dec. 31, 2042
Debt Instrument, Interest Rate, Stated Percentage	6.20%
Subordinated Debentures Due 2056 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 400,000,000	400,000,000
Debt Instrument, Face Amount	$ 400,000,000
Debt Instrument, Maturity Date	Dec. 31, 2056
Debt Instrument, Interest Rate, Stated Percentage	5.75%
Junior Subordinated Debentures Due 2065 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 318,743,000	318,740,000
Debt Instrument, Face Amount	$ 400,000,000
Debt Instrument, Maturity Date	Dec. 31, 2065
Debt Instrument, Interest Rate, Stated Percentage	4.82%
Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 89,098,000	$ 91,787,000
Debt Instrument, Face Amount	$ 100,000,000
Debt Instrument, Maturity Date	Dec. 31, 2039
Debt Instrument, Interest Rate, Stated Percentage	4.09%