Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 45,838	$ 86,686
Investment securities available for sale
Net unrealized gains (losses)	26,419	30,490
Amortization of net unrealized (gains) losses on available for sale securities transferred to held to maturity securities	556	69
Reclassification adjustment for net losses (gains) realized in net income	(6,118)	(1,680)
Income tax (expense) benefit	(5,225)	(7,301)
Other comprehensive income (loss) on investment securities available for sale	15,632	21,578
Defined benefit pension and postretirement obligations
Amortization of prior service cost	(38)	(38)
Amortization of actuarial loss (gain)	808	64
Income tax (expense) benefit	(193)	(7)
Other comprehensive income (loss) on pension and postretirement obligations	577	20
Total other comprehensive income (loss)	16,209	21,597
Comprehensive income	$ 62,046	$ 108,283

Amortization of Net Unrealized Gain on Available-for-Sale Securities Transfered to Held-to-maturity Securities

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