Investments (Components of Net Investment Income) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net Investment Income [Line Items]
Investment income, gross									$ 1,704,201	$ 1,545,892	$ 1,397,327
Less investment expenses									(12,009)	(14,225)	(13,400)
Net investment income	$ 438,262	$ 436,085	$ 418,176	$ 399,669	$ 403,849	$ 386,931	$ 370,882	$ 370,005	1,692,192	1,531,667	1,383,927
Fixed Maturity Securities [Member]
Net Investment Income [Line Items]
Investment income, gross									1,566,409	1,394,301	1,229,486
Equity Securities [Member]
Net Investment Income [Line Items]
Investment income, gross									441	404	1,586
Mortgage Loans on Real Estate [Member]
Net Investment Income [Line Items]
Investment income, gross									131,892	143,998	159,769
Cash and Cash Equivalents [Member]
Net Investment Income [Line Items]
Investment income, gross									601	286	775
Other [Member]
Net Investment Income [Line Items]
Investment income, gross									$ 4,858	$ 6,903	$ 5,711