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Fair Value Measurements (Carrying Value and Estimated Fair Value of Financial Instruments) (Details) - USD ($) $ in Thousands | Jun. 30, 2016 | Dec. 31, 2015
--- | --- | ---
Carrying Amount [Member] |  |
Held-to-maturity securities | $ 160,297 | $ 201,354
Available-for-sale securities | 4,671,786 | 4,408,203
Equity securities, AFS | 152,938 | 207,051
Short-term investments | 205,451 | 194,819
Short-term debt | 60,000 | 60,000
Long-term debt | 328,433 | 328,192
Carrying Amount [Member] | Seven Point Twenty Five Percent Senior Notes [Member] |  |
Long-term debt | 49,900 | 49,898
Carrying Amount [Member] | Six Point Seventy Percent Senior Notes [Member] |  |
Long-term debt | 99,422 | 99,415
Carrying Amount [Member] | Five Point Eight Hundred Seventy Five Senior Notes [Member] |  |
Long-term debt | 185,000 | 185,000
Carrying Amount [Member] | Long Term Debt Excluding Issuance Costs [Member] |  |
Long-term debt | 334,322 | 334,313
Carrying Amount [Member] | Unamortized Debt Issuance Costs [Member] |  |
Unamortized debt issuance costs | (5,889) | (6,121)
Carrying Amount [Member] | Zero Point Sixty Three Percent Borrowing From Federal Home Loan Bank Due July 22, 2016 [Member] |  |
Short-term debt | 15,000 | 15,000
Carrying Amount [Member] | One Point Twenty Five Percent Borrowing From Federal Home Loan Bank [Member] |  |
Short-term debt | 45,000 | 45,000
Fair Value [Member] |  |
Held-to-maturity securities | 167,061 | 209,544
Available-for-sale securities | 4,671,786 | 4,408,203
Equity securities, AFS | 152,938 | 207,051
Short-term investments | 205,451 | 194,819
Short-term debt | 60,114 | 60,060
Long-term debt | 373,971 | 359,766
Fair Value [Member] | Seven Point Twenty Five Percent Senior Notes [Member] |  |
Long-term debt | 61,003 | 56,929
Fair Value [Member] | Six Point Seventy Percent Senior Notes [Member] |  |
Long-term debt | 118,792 | 110,363
Fair Value [Member] | Five Point Eight Hundred Seventy Five Senior Notes [Member] |  |
Long-term debt | 194,176 | 192,474
Fair Value [Member] | Zero Point Sixty Three Percent Borrowing From Federal Home Loan Bank Due July 22, 2016 [Member] |  |
Short-term debt | 15,001 | 14,977
Fair Value [Member] | One Point Twenty Five Percent Borrowing From Federal Home Loan Bank [Member] |  |
Short-term debt | $ 45,113 | $ 45,083

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