Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$ 38,093	$ 22,549
Other comprehensive income, net of tax
Unrealized gains on securities	(370)	8,748
Pension and other postretirement benefits	947	933
Other comprehensive income (loss), net of tax	577	9,681
Comprehensive income	$ 38,670	$ 32,230

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Other Comprehensive Income
-URI http://asc.fasb.org/extlink&oid=51831270

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Comprehensive Income
-URI http://asc.fasb.org/extlink&oid=51831223

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e557-108580

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=51831255

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Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Amount after tax of other comprehensive income (loss) attributable to parent entity.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 1A
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Amount after tax of reclassification adjustment from accumulated other comprehensive (income) loss related to net period benefit cost (credit) for pension and other postretirement defined benefit plans.

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Amount after tax, before reclassification adjustments, of unrealized holding gain (loss) on available-for-sale securities.

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