Investments (Details 3) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2014		Dec. 31, 2013
Estimated expected maturity:
Due in 1 year or less, Amortized Cost	$ 262,082
Due in 1 year or less, Fair Value	278,051
Due in 1 year or less, Percent of Total Fair Value	4.30%		4.10%
Due after 1 year through 5 years, Amortized Cost	1,319,172
Due after 1 year through 5 years, Fair Value	1,399,549
Due after 1 year through 5 years, Percent of Total Fair Value	21.90%		20.90%
Due after 5 years through 10 years, Amortized Cost	2,379,748
Due after 5 years through 10 years, Fair Value	2,524,744
Due after 5 years through 10 years, Percent of Total Fair Value	39.50%		38.40%
Due after 10 years through 20 years, Amortized Cost	1,198,682
Due after 10 years through 20 years, Fair Value	1,271,718
Due after 10 years through 20 years, Percent of Total Fair Value	19.90%		20.80%
Due after 20 years, Amortized Cost	871,292
Due after 20 years, Fair Value	924,380
Due after 20 years, Percent of Total Fair Value	14.40%		15.80%
Total, Amortized Cost	6,030,976		5,784,205
Fair Value	$ 6,398,442	[1]	$ 6,009,573	[1]
Total, Percent of Total Fair Value	100.00%		100.00%
Average option-adjusted duration, in years	6 years 1 month 6 days		6 years 3 months 18 days

[1] Fair value includes securities issued by Federal National Mortgage Association (“FNMA”) of $343,090 and $336,193; Federal Home Loan Mortgage Corporation (“FHLMC”) of $434,214 and $427,172; and Government National Mortgage Association (“GNMA”) of $132,896 and $126,245 as of March 31, 2014 and December 31, 2013, respectively.