Shareholders' Equity - Reclassifications from AOCI to Net Income (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification adjustments related to available-for-sale securities
Total gains (losses) reclassified, before-tax effect	$ 2,509	$ (2,724)	$ (6,754)
Tax effect (at 35%)	(878)	953	2,364
Net reclassification adjustments	1,631	(1,771)	(4,390)
Deferred tax expense (benefit) included in OCI	(4,676)	(3,078)	(18,370)
Realized investment gains (losses)
Reclassification adjustments related to available-for-sale securities
Total gains (losses) reclassified, before-tax effect	2,512	2,417	(4,475)
Non-credit impairment losses reclassified to earnings, due to sale of securities or reclassification as a credit loss
Reclassification adjustments related to available-for-sale securities
Total gains (losses) reclassified, before-tax effect	(3)	(3,641)	(2,279)
Unrecognized losses in defined benefit plan liabilities reclassified to earnings, due to the termination and settlement of the plan
Reclassification adjustments related to available-for-sale securities
Total gains (losses) reclassified, before-tax effect	$ 0	$ (1,500)	$ 0

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Amount of tax expense (benefit) allocated to other comprehensive income (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e640-108580

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 20
-Section 45
-Paragraph 11
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=79507043&loc=d3e39076-109324

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Amount before tax of reclassification adjustments of other comprehensive income (loss).

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Amount after tax of reclassification adjustments of other comprehensive income (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 55
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=84158357&loc=d3e1436-108581

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Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 20
-Section 45
-Paragraph 11
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=79507043&loc=d3e39076-109324

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e640-108580

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