Fair Value Measurements (Carrying Value and Estimated Fair Value of Financial Instruments) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fixed income securities, held-to-maturity	$ 37,110	$ 42,129
Debt Securities, Available-for-sale	5,273,100	5,162,522
Equity securities	147,639
Equity securities, AFS		182,705
Short-term investments	323,864	165,555
Long-term debt	439,540	439,116
Seven Point Twenty Five Percent Senior Notes [Member]
Long-term Debt, Fair Value	57,032	61,391
Six Point Seventy Percent Senior Notes [Member]
Long-term Debt, Fair Value	107,075	116,597
Five Point Eight Hundred Seventy Five Percent Senior Notes [Member]
Long-term Debt, Fair Value	177,230	186,332
One Point Sixty One Percent Borrowing From Federal Home Loan Bank Due July 21, 2021 [Member]
Long-term Debt, Fair Value	24,218	24,270
One Point Fifty Six Percent Borrowing From Federal Home Loan Bank Due August 16, 2021 [Member]
Long-term Debt, Fair Value	24,162	24,210
Three Point Zero Three Percent Borrowing From Federal Home Loan Bank Due December 16, 2026 [Member]
Long-term Debt, Fair Value	58,905	60,334
Carrying Amount [Member]
Fixed income securities, held-to-maturity	37,110	42,129
Debt Securities, Available-for-sale	5,273,100	5,162,522
Equity securities	147,639
Equity securities, AFS		182,705
Short-term investments	323,864	165,555
Long-term debt	439,540	439,116
Carrying Amount [Member] \| Seven Point Twenty Five Percent Senior Notes [Member]
Long-term debt	49,907	49,904
Carrying Amount [Member] \| Six Point Seventy Percent Senior Notes [Member]
Long-term debt	99,462	99,446
Carrying Amount [Member] \| Five Point Eight Hundred Seventy Five Percent Senior Notes [Member]
Long-term debt	185,000	185,000
Carrying Amount [Member] \| One Point Sixty One Percent Borrowing From Federal Home Loan Bank Due July 21, 2021 [Member]
Long-term debt	25,000	25,000
Carrying Amount [Member] \| One Point Fifty Six Percent Borrowing From Federal Home Loan Bank Due August 16, 2021 [Member]
Long-term debt	25,000	25,000
Carrying Amount [Member] \| Three Point Zero Three Percent Borrowing From Federal Home Loan Bank Due December 16, 2026 [Member]
Long-term debt	60,000	60,000
Carrying Amount [Member] \| Long Term Debt Excluding Issuance Costs [Member]
Long-term debt	444,369	444,350
Carrying Amount [Member] \| Unamortized Debt Issuance Costs [Member]
Unamortized debt issuance costs	(4,829)	(5,234)
Fair Value [Member]
Fixed income securities, held-to-maturity	38,317	44,100
Debt Securities, Available-for-sale	5,273,100	5,162,522
Equity securities	147,639
Equity securities, AFS		182,705
Short-term investments	323,864	165,555
Long-term Debt, Fair Value	448,622	473,134
Fair Value [Member] \| Seven Point Twenty Five Percent Senior Notes [Member]
Long-term Debt, Fair Value	57,032	61,391
Fair Value [Member] \| Six Point Seventy Percent Senior Notes [Member]
Long-term Debt, Fair Value	107,075	116,597
Fair Value [Member] \| Five Point Eight Hundred Seventy Five Percent Senior Notes [Member]
Long-term Debt, Fair Value	177,230	186,332
Fair Value [Member] \| One Point Sixty One Percent Borrowing From Federal Home Loan Bank Due July 21, 2021 [Member]
Long-term Debt, Fair Value	24,218	24,270
Fair Value [Member] \| One Point Fifty Six Percent Borrowing From Federal Home Loan Bank Due August 16, 2021 [Member]
Long-term Debt, Fair Value	24,162	24,210
Fair Value [Member] \| Three Point Zero Three Percent Borrowing From Federal Home Loan Bank Due December 16, 2026 [Member]
Long-term Debt, Fair Value	$ 58,905	$ 60,334

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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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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Amount of short-term investments classified as other.

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