Stockholders Equity Accumulated other comprehensive income (Details) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016		Dec. 31, 2016		Dec. 31, 2015
Accumulated other comprehensive income		$ 223,869	$ 292,828		$ 149,555		$ 114,532
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, before Reclassification Adjustments, Net of Tax		73,813	178,873
Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated other comprehensive income		229,195	296,291		156,963	[1]	120,787	[1]
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, before Reclassification Adjustments, Net of Tax	[1]	72,293	175,562
Accumulated Other-than-Temporary Impairment [Member]
Accumulated other comprehensive income		1,831	2,245		311		(114)
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, before Reclassification Adjustments, Net of Tax		1,520	3,311
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated other comprehensive income		(7,157)	(5,708)		$ (7,719)		$ (6,141)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, before Reclassification Adjustments, Net of Tax		0	0
reclassifications out of accumulated other comprehensive income [Member]
Other Comprehensive Income (Loss), Reclassification Adjustment for Sale of Securities Included in Net Income, Net of Tax		501	(577)
reclassifications out of accumulated other comprehensive income [Member] | Accumulated Net Unrealized Investment Gain (Loss) [Member]
Other Comprehensive Income (Loss), Reclassification Adjustment for Sale of Securities Included in Net Income, Net of Tax	[1]	(61)	(58)	[2]
reclassifications out of accumulated other comprehensive income [Member] | Accumulated Other-than-Temporary Impairment [Member]
Other Comprehensive Income (Loss), Reclassification Adjustment for Sale of Securities Included in Net Income, Net of Tax	[2]	0	(952)
reclassifications out of accumulated other comprehensive income [Member] | Accumulated Defined Benefit Plans Adjustment [Member]
Other Comprehensive Income (Loss), Reclassification, Pension and Other Postretirement Benefit Plans, Net Gain (Loss) Recognized in Net Periodic Benefit Cost, Net of Tax		$ 562	$ 433

[1] Includes the impact of taxes, deferred acquisition costs, value of insurance in force acquired, unearned revenue reserves and policyholder liabilities. See Note 2 for further information.

[2] See Note 2 for further information.