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Fair Value Measurements (Carrying Value and Estimated Fair Value of Financial Instruments) (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Held-to-maturity Securities | $ 42,129 | $ 101,556
Available-for-sale Securities, Debt Securities | 5,162,522 | 4,792,540
Equity securities, AFS | 182,705 | 146,753
Long-term debt | 439,116 | 438,667
Short-term investments | 165,555 | 221,701
Seven Point Twenty Five Percent Senior Notes [Member] |  |
Long-term Debt, Fair Value | 61,391 | 56,148
Six Point Seventy Percent Senior Notes [Member] |  |
Long-term Debt, Fair Value | 116,597 | 108,333
Five Point Eight Hundred Seventy Five Percent Senior Notes [Member] |  |
Long-term Debt, Fair Value | 186,332 | 176,860
One Point Sixty One Percent Borrowing From Federal Home Loan Bank Due July 21, 2021 [Member] |  |
Long-term Debt, Fair Value | 24,270 | 24,286
One Point Fifty Six Percent Borrowing From Federal Home Loan Bank Due August 16, 2021 [Member] |  |
Long-term Debt, Fair Value | 24,210 | 24,219
Three Point Zero Three Percent Borrowing From Federal Home Loan Bank Due December 16, 2026 [Member] |  |
Long-term Debt, Fair Value | 60,334 | 59,313
Carrying Amount [Member] |  |
Held-to-maturity Securities | 42,129 | 101,556
Available-for-sale Securities, Debt Securities | 5,162,522 | 4,792,540
Equity securities, AFS | 182,705 | 146,753
Long-term debt | 439,116 | 438,667
Short-term investments | 165,555 | 221,701
Carrying Amount [Member] | Seven Point Twenty Five Percent Senior Notes [Member] |  |
Long-term debt | 49,904 | 49,901
Carrying Amount [Member] | Six Point Seventy Percent Senior Notes [Member] |  |
Long-term debt | 99,446 | 99,430
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 | 444,350 | 444,331
Carrying Amount [Member] | Unamortized Debt Issuance Costs [Member] |  |
Unamortized Debt Issuance Costs Related to Long-Term Debt | (5,234) | (5,664)
Fair Value [Member] |  |
Held-to-maturity Securities | 44,100 | 105,211
Available-for-sale Securities, Debt Securities | 5,162,522 | 4,792,540
Equity securities, AFS | 182,705 | 146,753
Long-term Debt, Fair Value | 473,134 | 449,159
Short-term investments | 165,555 | 221,701
Fair Value [Member] | Seven Point Twenty Five Percent Senior Notes [Member] |  |
Long-term Debt, Fair Value | 61,391 | 56,148
Fair Value [Member] | Six Point Seventy Percent Senior Notes [Member] |  |
Long-term Debt, Fair Value | 116,597 | 108,333
Fair Value [Member] | Five Point Eight Hundred Seventy Five Percent Senior Notes [Member] |  |
Long-term Debt, Fair Value | 186,332 | 176,860
Fair Value [Member] | One Point Sixty One Percent Borrowing From Federal Home Loan Bank Due July 21, 2021 [Member] |  |
Long-term Debt, Fair Value | 24,270 | 24,286
Fair Value [Member] | One Point Fifty Six Percent Borrowing From Federal Home Loan Bank Due August 16, 2021 [Member] |  |
Long-term Debt, Fair Value | 24,210 | 24,219
Fair Value [Member] | Three Point Zero Three Percent Borrowing From Federal Home Loan Bank Due December 16, 2026 [Member] |  |
Long-term Debt, Fair Value | $ 60,334 | $ 59,313

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