Comprehensive Income (Loss) (Reclassification From Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands	3 Months Ended					6 Months Ended		9 Months Ended	12 Months Ended
	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2017	Jun. 30, 2016	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other net realized capital gains (losses)	$ (90,717)			$ 30,556		$ 28,183	$ (12,806)
Income tax expense (benefit)	8,601	$ (75,769)	$ (22,425)	(33,677)	$ (13,256)	(67,168)	(46,933)	$ (69,358)	$ (97,347)	$ (194,896)	$ (204,687)
Net income (loss)	35,658	$ 169,178	$ 76,303	69,372	$ 40,990	204,836	110,362	$ 186,665	$ 301,644	$ 412,390	$ 453,428
URA(D) On Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other underwriting expenses	(4,931)			(97)		(10,265)	39,773
Income tax expense (benefit)	1,454			34		3,321	(13,921)
Net income (loss)	(3,477)			(63)		(6,944)	25,852
URA(D) On Securities [Member] | Reclassification From Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other net realized capital gains (losses)	(4,931)			(97)		(10,265)	39,773
Income tax expense (benefit)	1,454			34		3,321	(13,921)
Net income (loss)	(3,477)			(63)		(6,944)	25,852
Benefit Plan Net Gain (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other underwriting expenses	3,083			2,063		6,166	4,125
Income tax expense (benefit)	(1,079)			(722)		(2,158)	(1,444)
Net income (loss)	$ 2,004			$ 1,341		$ 4,008	$ 2,681