Investments (Details 3) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2014	Dec. 31, 2013
Estimated expected maturity:
Due in 1 year or less, Amortized Cost	$ 277,477
Due in 1 year or less, Fair Value	299,402
Due in 1 year or less, Percent of Total Fair Value	4.50%	4.10%
Due after 1 year through 5 years, Amortized Cost	1,461,075
Due after 1 year through 5 years, Fair Value	1,576,524
Due after 1 year through 5 years, Percent of Total Fair Value	23.60%	20.90%
Due after 5 years through 10 years, Amortized Cost	2,355,992
Due after 5 years through 10 years, Fair Value	2,542,155
Due after 5 years through 10 years, Percent of Total Fair Value	38.10%	38.40%
Due after 10 years through 20 years, Amortized Cost	1,230,271
Due after 10 years through 20 years, Fair Value	1,327,483
Due after 10 years through 20 years, Percent of Total Fair Value	19.90%	20.80%
Due after 20 years, Amortized Cost	862,614
Due after 20 years, Fair Value	930,775
Due after 20 years, Percent of Total Fair Value	13.90%	15.80%
Total, Amortized Cost	6,187,429	5,784,205
Fair Value	$ 6,676,339	$ 6,009,573
Total, Percent of Total Fair Value	100.00%	100.00%
Average option-adjusted duration, in years	6 years 2 months 12 days	6 years 3 months 18 days