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Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017
Net income | $ 42,437 | $ 37,325
Derivative instruments: |  |
Reclassification adjustment for loss on derivative instruments included in net income, net of tax of $56 and $56 for the three months ended in 2018 and 2017, respectively | 92 | 91
Postretirement liability adjustment: |  |
Amortization of postretirement liability losses included in net periodic benefit cost (credit), net of tax of $155 and $217 for the three months ended in 2018 and 2017, respectively | 418 | 357
Reclassification of postretirement liability adjustment from regulatory asset, net of tax of $0 and $(725) for the three months ended in 2018 and 2017, respectively | 0 | (917)
Postretirement liability adjustment | 418 | (560)
Foreign currency translation adjustment: |  |
Foreign currency translation adjustment recognized during the period, net of tax of $(1) and $5 for the three months ended in 2018 and 2017, respectively | (2) | 9
Net unrealized gain (loss) on available-for-sale investments: |  |
Net unrealized loss on available-for-sale investments arising during the period, net of tax of $(28) and $(15) for the three months ended in 2018 and 2017, respectively | (105) | (27)
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $7 and $19 for the three months ended in 2018 and 2017, respectively | 30 | 35
Net unrealized gain (loss) on available-for-sale investments | (75) | 8
Other comprehensive income (loss) | 433 | (452)
Comprehensive income attributable to common stockholders | $ 42,870 | $ 36,873

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