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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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