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Fair Value Measurements (Carrying Value and Estimated Fair Value of Financial Instruments) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Held-to-maturity securities	$ 130,472	$ 201,354
Available-for-sale securities	4,832,532	4,408,203
Equity securities, AFS	147,304	207,051
Short-term investments	169,604	194,819
Short-term debt	45,000	60,000
Long-term debt	378,551	328,192
Seven Point Twenty Five Percent Senior Notes [Member]
Long-term debt	60,682	56,929
Six Point Seventy Percent Senior Notes [Member]
Long-term debt	118,192	110,363
Five Point Eight Hundred Seventy Five Senior Notes [Member]
Long-term debt	194,324	192,474
One Point Sixty One Percent Borrowing From Federal Home Loan Bank Due July 21, 2021 [Member]
Long-term debt	25,048
One Point Fifty Six Percent Borrowing From Federal Home Loan Bank Due August 16, 2021 [Member]
Long-term debt	24,990
Zero Point Sixty Three Percent Borrowing From Federal Home Loan Bank Due July 22, 2016 [Member]
Short-term debt		14,977
One Point Twenty Five Percent Borrowing From Federal Home Loan Bank [Member]
Short-term debt	45,072	45,083
Reported Value Measurement [Member]
Held-to-maturity securities	130,472	201,354
Available-for-sale securities	4,832,532	4,408,203
Equity securities, AFS	147,304	207,051
Short-term investments	169,604	194,819
Short-term debt	45,000	60,000
Long-term debt	378,551	328,192
Reported Value Measurement [Member] | Seven Point Twenty Five Percent Senior Notes [Member]
Long-term debt	49,900	49,898
Reported Value Measurement [Member] | Six Point Seventy Percent Senior Notes [Member]
Long-term debt	99,426	99,415
Reported Value Measurement [Member] | Five Point Eight Hundred Seventy Five Senior Notes [Member]
Long-term debt	185,000	185,000
Reported Value Measurement [Member] | One Point Sixty One Percent Borrowing From Federal Home Loan Bank Due July 21, 2021 [Member]
Long-term debt	25,000	0
Reported Value Measurement [Member] | One Point Fifty Six Percent Borrowing From Federal Home Loan Bank Due August 16, 2021 [Member]
Long-term debt	25,000	0
Reported Value Measurement [Member] | Long Term Debt Excluding Issuance Costs [Member]
Long-term debt	384,326	334,313
Reported Value Measurement [Member] | Unamortized Debt Issuance Costs [Member]
Unamortized debt issuance costs	(5,775)	(6,121)
Reported Value Measurement [Member] | Zero Point Sixty Three Percent Borrowing From Federal Home Loan Bank Due July 22, 2016 [Member]
Short-term debt	0	15,000
Reported Value Measurement [Member] | One Point Twenty Five Percent Borrowing From Federal Home Loan Bank [Member]
Short-term debt	45,000	45,000
Estimate of Fair Value Measurement [Member]
Held-to-maturity securities	136,094	209,544
Available-for-sale securities	4,832,532	4,408,203
Equity securities, AFS	147,304	207,051
Short-term investments	169,604	194,819
Short-term debt	45,072	60,060
Long-term debt	423,236	359,766
Estimate of Fair Value Measurement [Member] | Seven Point Twenty Five Percent Senior Notes [Member]
Long-term debt	60,682	56,929
Estimate of Fair Value Measurement [Member] | Six Point Seventy Percent Senior Notes [Member]
Long-term debt	118,192	110,363
Estimate of Fair Value Measurement [Member] | Five Point Eight Hundred Seventy Five Senior Notes [Member]
Long-term debt	194,324	192,474
Estimate of Fair Value Measurement [Member] | One Point Sixty One Percent Borrowing From Federal Home Loan Bank Due July 21, 2021 [Member]
Long-term debt	25,048	0
Estimate of Fair Value Measurement [Member] | One Point Fifty Six Percent Borrowing From Federal Home Loan Bank Due August 16, 2021 [Member]
Long-term debt	24,990	0
Estimate of Fair Value Measurement [Member] | Zero Point Sixty Three Percent Borrowing From Federal Home Loan Bank Due July 22, 2016 [Member]
Short-term debt	0	14,977
Estimate of Fair Value Measurement [Member] | One Point Twenty Five Percent Borrowing From Federal Home Loan Bank [Member]
Short-term debt	$ 45,072	$ 45,083

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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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