Fair Value Measurements (Carrying Value and Estimated Fair Value of Financial Instruments) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Held-to-maturity Securities	$ 247,859	$ 318,137
Available-for-sale Securities	4,192,818	4,066,122
Equity securities, AFS	228,883	191,400
Short-term investments	168,349	131,972
Notes payable	394,305	379,297
Carrying Amount [Member]
Held-to-maturity Securities	247,859	318,137
Available-for-sale Securities	4,192,818	4,066,122
Equity securities, AFS	228,883	191,400
Short-term investments	168,349	131,972
Notes payable	394,305	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	49,897	49,896
Carrying Amount [Member] | Six Point Seventy Percent Senior Notes [Member]
Notes payable	99,408	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	259,702	333,961
Available-for-sale Securities	4,192,818	4,066,122
Equity securities, AFS	228,883	191,400
Short-term investments	168,349	131,972
Notes payable	410,294	404,270
Fair Value [Member] | Zero Point Sixty Three Percent Borrowing From Federal Home Loan Bank Due July 22, 2016 [Member]
Notes payable	14,999	0
Fair Value [Member] | One Point Twenty Five Percent Borrowing From Federal Home Loan Bank [Member]
Notes payable	45,290	45,244
Fair Value [Member] | Seven Point Twenty Five Percent Senior Notes [Member]
Notes payable	56,607	59,181
Fair Value [Member] | Six Point Seventy Percent Senior Notes [Member]
Notes payable	109,656	114,845
Fair Value [Member] | Five Point Eight Hundred Seventy Five Senior Notes [Member]
Notes payable	$ 183,742	$ 185,000