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Comprehensive Income (Components of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance, December 31, 2017	$ 20,170
Amounts reclassified into net income: Non-credit OTTI	0
Amounts reclassified from AOCI, HTM related	(10)	$ (32)
Amounts reclassified from AOCI, All Other	3,594	981
OCI before reclassifications, Defined Benefit Pension and Post Retirement Plans	0
Amount reclassified into net income: Net actuarial loss	420	330
Other comprehensive income	(63,394)	18,040
Other Comprehensive Income, Defined Benefit Pension and Post Retirement Plans, Adjustment, Net of Tax	420	330
Ending Balance, March 31, 2018	(68,243)
Available-for-sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance, December 31, 2017	80,648
Cumulative effect of new accounting principle in period of adoption	(12,792)
OCI before reclassifications	(67,398)
Other comprehensive income	(63,804)
Ending Balance, March 31, 2018	4,052
Investments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance, December 31, 2017	80,575
Cumulative effect of new accounting principle in period of adoption	(12,806)
OCI before reclassifications	(67,398)
Amounts reclassified from AOCI	3,584
Other comprehensive income	(63,814)
Ending Balance, March 31, 2018	3,955
Accumulated Other-than-Temporary Impairment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance, December 31, 2017	(59)
Cumulative effect of new accounting principle in period of adoption	(12)
OCI before reclassifications	0
Other comprehensive income	0
Ending Balance, March 31, 2018	(71)
Accumulated Net Unrealized Investment Gain (Loss) [Member] | Held-to-maturity Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance, December 31, 2017	(14)
Cumulative effect of new accounting principle in period of adoption	(2)
OCI before reclassifications	0
Other comprehensive income	(10)
Ending Balance, March 31, 2018	(26)
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance, December 31, 2017	(60,405)
Cumulative effect of new accounting principle in period of adoption	(12,213)
Amount reclassified into net income: Net actuarial loss	420
Ending Balance, March 31, 2018	(72,198)
Accumulated other comprehensive income (loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance, December 31, 2017	20,170
Cumulative effect of new accounting principle in period of adoption	(25,019)
OCI before reclassifications	(67,398)
Amounts reclassified from AOCI	4,004
Other comprehensive income	(63,394)	$ 18,040
Ending Balance, March 31, 2018	(68,243)
Adjustments for New Accounting Pronouncement [Member] | Available-for-sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance, December 31, 2017	67,856
Adjustments for New Accounting Pronouncement [Member] | Investments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance, December 31, 2017	67,769
Adjustments for New Accounting Pronouncement [Member] | Accumulated Other-than-Temporary Impairment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance, December 31, 2017	(71)
Adjustments for New Accounting Pronouncement [Member] | Accumulated Net Unrealized Investment Gain (Loss) [Member] | Held-to-maturity Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance, December 31, 2017	(16)
Adjustments for New Accounting Pronouncement [Member] | Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance, December 31, 2017	(72,618)
Adjustments for New Accounting Pronouncement [Member] | Accumulated other comprehensive income (loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance, December 31, 2017	$ (4,849)

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