Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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

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

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Amount of unamortized debt discount (premium) and debt issuance costs.

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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, 2016	Dec. 31, 2015	Jun. 30, 2006
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 3,110,853,000	$ 2,312,629,000
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	22,218,000	15,081,000
Debt Instrument, Face Amount			$ 850,000,000.0
Long-term debt	3,088,635,000	2,297,548,000
Senior Notes Due 2017 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	299,945,000	299,671,000
Debt Instrument, Face Amount	$ 300,000,000.0
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	$ 399,805,000	399,737,000
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	$ 399,025,000	398,803,000
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	$ 398,986,000	398,835,000
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	$ 399,985,000	0
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	8,800,000
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	0
Debt Instrument, Face Amount	$ 400,000,000.0
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	$ 318,737,000	318,734,000
Debt Instrument, Face Amount	$ 400,000,000
Debt Instrument, Maturity Date	Dec. 31, 2065
Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 94,370,000	$ 96,849,000
Debt Instrument, Face Amount	$ 100,000,000
Debt Instrument, Maturity Date	Dec. 31, 2039
Debt Instrument, Interest Rate, Stated Percentage	4.09%