Fair Value Measurements (Carrying Value and Estimated Fair Value of Financial Instruments) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Carrying Amount [Member]
Held-to-maturity Securities	$ 229,869	$ 318,137
Available-for-sale Securities	4,351,046	4,066,122
Equity securities, AFS	221,951	191,400
Short-term investments	125,855	131,972
Notes payable	394,309	379,297
Carrying Amount [Member] | Zero Point Sixty Three Percent Borrowing From Federal Home Loan Bank Due July 22, 2016 [Member]
Notes payable	15,000	0
Carrying Amount [Member] | One Point Twenty Five Percent Borrowing From Federal Home Loan Bank [Member]
Notes payable	45,000	45,000
Carrying Amount [Member] | Seven Point Twenty Five Percent Senior Notes [Member]
Notes payable	49,898	49,896
Carrying Amount [Member] | Six Point Seventy Percent Senior Notes [Member]
Notes payable	99,411	99,401
Carrying Amount [Member] | Five Point Eight Hundred Seventy Five Senior Notes [Member]
Notes payable	185,000	185,000
Fair Value [Member]
Held-to-maturity Securities	240,388	333,961
Available-for-sale Securities	4,351,046	4,066,122
Equity securities, AFS	221,951	191,400
Short-term investments	125,855	131,972
Notes payable	418,548	404,270
Fair Value [Member] | Zero Point Sixty Three Percent Borrowing From Federal Home Loan Bank Due July 22, 2016 [Member]
Notes payable	14,995	0
Fair Value [Member] | One Point Twenty Five Percent Borrowing From Federal Home Loan Bank [Member]
Notes payable	45,277	45,244
Fair Value [Member] | Seven Point Twenty Five Percent Senior Notes [Member]
Notes payable	57,929	59,181
Fair Value [Member] | Six Point Seventy Percent Senior Notes [Member]
Notes payable	112,387	114,845
Fair Value [Member] | Five Point Eight Hundred Seventy Five Senior Notes [Member]
Notes payable	$ 187,960	$ 185,000