Securities - Summary of Book Values and Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Securities available for sale:
Amortized Cost	$ 1,432,684	$ 812,202
Fair Value	1,453,132	821,945
Unrealized Gains	22,963	10,505
Unrealized (Losses)	(2,515)	(762)
Securities held to maturity:
Amortized Cost	167,551	67,932
Fair Value	170,734	68,333
Unrealized Gains	3,348	410
Unrealized (Losses)	(165)	(9)
Government-sponsored enterprise securities
Securities available for sale:
Amortized Cost	70,016	20,000
Fair Value	70,206	20,009
Unrealized Gains	371	17
Unrealized (Losses)	(181)	(8)
Mortgage-backed securities
Securities available for sale:
Amortized Cost	1,318,998	758,491
Fair Value	1,337,706	767,285
Unrealized Gains	20,832	9,463
Unrealized (Losses)	(2,124)	(669)
Securities held to maturity:
Amortized Cost	29,959	41,423
Fair Value	30,900	41,542
Unrealized Gains	941	125
Unrealized (Losses)	0	(6)
Corporate bonds
Securities available for sale:
Amortized Cost	43,670	33,711
Fair Value	45,220	34,651
Unrealized Gains	1,760	1,025
Unrealized (Losses)	(210)	(85)
State and local governments
Securities held to maturity:
Amortized Cost	137,592	26,509
Fair Value	139,834	26,791
Unrealized Gains	2,407	285
Unrealized (Losses)	$ (165)	$ (3)

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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