Investments (Other Than Temporary Impairment by Asset Type) (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) Securities	Dec. 31, 2016 USD ($) Securities	Dec. 31, 2015 USD ($) Securities
Gain (Loss) on Investments [Line Items]
Other than temporary impairment, number of securities | Securities									10	22	14
Total OTTI losses									$ (2,758)	$ (21,349)	$ (25,547)
Portion of OTTI losses recognized in (from) other comprehensive income									(1,872)	(1,330)	6,011
Net OTTI losses recognized in operations	$ (3,076)	$ (464)	$ (949)	$ (141)	$ (9,560)	$ (2,979)	$ (4,446)	$ (5,694)	$ (4,630)	$ (22,679)	$ (19,536)
Corporate Securities [Member] | Energy [Member]
Gain (Loss) on Investments [Line Items]
Other than temporary impairment, number of securities | Securities										2
Total OTTI losses										$ (642)
Portion of OTTI losses recognized in (from) other comprehensive income										0
Net OTTI losses recognized in operations										$ (642)
Corporate Securities [Member] | Industrial [Member]
Gain (Loss) on Investments [Line Items]
Other than temporary impairment, number of securities | Securities									1		2
Total OTTI losses									$ (2,485)		$ (15,414)
Portion of OTTI losses recognized in (from) other comprehensive income									0		2,975
Net OTTI losses recognized in operations									$ (2,485)		$ (12,439)
Corporate Securities [Member] | Materials [Member]
Gain (Loss) on Investments [Line Items]
Other than temporary impairment, number of securities | Securities										1
Total OTTI losses										$ (4,554)
Portion of OTTI losses recognized in (from) other comprehensive income										1,575
Net OTTI losses recognized in operations										$ (2,979)
Corporate Securities [Member] | Telecommunications [Member]
Gain (Loss) on Investments [Line Items]
Other than temporary impairment, number of securities | Securities										1
Total OTTI losses										$ (4,462)
Portion of OTTI losses recognized in (from) other comprehensive income										562
Net OTTI losses recognized in operations										$ (3,900)
Corporate Securities [Member] | Utilities [Member]
Gain (Loss) on Investments [Line Items]
Other than temporary impairment, number of securities | Securities										2
Total OTTI losses										$ (6,961)
Portion of OTTI losses recognized in (from) other comprehensive income										798
Net OTTI losses recognized in operations										$ (6,163)
Residential Mortgage Backed Securities [Member]
Gain (Loss) on Investments [Line Items]
Other than temporary impairment, number of securities | Securities									8	9	11
Total OTTI losses									$ (273)	$ 0	$ (133)
Portion of OTTI losses recognized in (from) other comprehensive income									(1,585)	(783)	(2,089)
Net OTTI losses recognized in operations									$ (1,858)	$ (783)	$ (2,222)
Commercial Mortgage Backed Securities [Member]
Gain (Loss) on Investments [Line Items]
Other than temporary impairment, number of securities | Securities										5
Total OTTI losses										$ (1,540)
Portion of OTTI losses recognized in (from) other comprehensive income										0
Net OTTI losses recognized in operations										$ (1,540)
Other Asset Backed Securities [Member]
Gain (Loss) on Investments [Line Items]
Other than temporary impairment, number of securities | Securities									1	2	1
Total OTTI losses									$ 0	$ (3,190)	$ (10,000)
Portion of OTTI losses recognized in (from) other comprehensive income									(287)	(3,482)	5,125
Net OTTI losses recognized in operations									$ (287)	$ (6,672)	$ (4,875)