Senior Notes, Convertible Notes and Trust Preferred Securities - Schedule of Long-term Debt Instruments (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Unamortized debt issuance costs	$ (5,094,000)
Senior Notes [Member]
Debt Instrument [Line Items]
Principal amount of debt component	500,000,000	$ 500,000,000
Unamortized debt discounts	(1,235,000)	(1,507,000)
Unamortized debt issuance costs	(2,731,000)	(3,295,000)
Long-term debt	496,034,000	495,198,000
Senior Notes [Member] \| Senior Notes Due 2024 [Member]
Debt Instrument [Line Items]
Principal amount of debt component	$ 250,000,000	250,000,000
Debt interest rate percentage	4.40%
Issue Price	99.883%
Senior Notes [Member] \| Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Principal amount of debt component	$ 250,000,000	$ 250,000,000
Debt interest rate percentage	4.25%
Issue Price	99.026%

Debt Instrument, Percentage of Issuance Price

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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, after accumulated amortization, of debt discount.

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration 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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