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Fair Value Measurements (Carrying Value and Estimated Fair Value of Financial Instruments) (Details) - USD ($) $ in Thousands | Mar. 31, 2016 | Dec. 31, 2015
--- | --- | ---
Carrying Amount [Member] |  |
Held-to-maturity securities | $ 163,573 | $ 201,354
Available-for-sale securities | 4,600,468 | 4,408,203
Equity securities, AFS | 215,789 | 207,051
Short-term investments | 103,132 | 194,819
Short-term debt | 60,000 | 60,000
Long-term debt | 328,313 | 328,192
Carrying Amount [Member] | Seven Point Twenty Five Percent Senior Notes [Member] |  |
Long-term debt | 49,899 | 49,898
Carrying Amount [Member] | Six Point Seventy Percent Senior Notes [Member] |  |
Long-term debt | 99,419 | 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 | 334,318 | 334,313
Carrying Amount [Member] | Unamortized Debt Issuance Costs [Member] |  |
Long-term debt | 6,005 | 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 | 171,259 | 209,544
Available-for-sale securities | 4,600,468 | 4,408,203
Equity securities, AFS | 215,789 | 207,051
Short-term investments | 103,132 | 194,819
Short-term debt | 60,264 | 60,060
Long-term debt | 367,136 | 359,766
Fair Value [Member] | Seven Point Twenty Five Percent Senior Notes [Member] |  |
Long-term debt | 59,406 | 56,929
Fair Value [Member] | Six Point Seventy Percent Senior Notes [Member] |  |
Long-term debt | 115,478 | 110,363
Fair Value [Member] | Five Point Eight Hundred Seventy Five Senior Notes [Member] |  |
Long-term debt | 192,252 | 192,474
Fair Value [Member] | Zero Point Sixty Three Percent Borrowing From Federal Home Loan Bank Due July 22, 2016 [Member] |  |
Short-term debt | 15,014 | 14,977
Fair Value [Member] | One Point Twenty Five Percent Borrowing From Federal Home Loan Bank [Member] |  |
Short-term debt | $ 45,250 | $ 45,083

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