Investment Securities, Securities Available-for-Sale (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2021	Sep. 30, 2021		Dec. 31, 2020	Sep. 30, 2020
Amortized cost, fair values, and unrealized gains and losses of securities available-for-sale [Abstract]
Amortized cost			$ 333,994		$ 789,175	$ 550,053
Gross unrealized gains			4,541		18,716	18,722
Gross unrealized losses			5,384		159	239
Fair value			333,151		807,732	568,536
U.S. Treasury Notes [Member]
Amortized cost, fair values, and unrealized gains and losses of securities available-for-sale [Abstract]
Amortized cost			9,922		14,859	94,830
Gross unrealized gains			228		429	512
Gross unrealized losses			0		0	0
Fair value			10,150		15,288	95,342
U.S. Government Agency SBA [Member]
Amortized cost, fair values, and unrealized gains and losses of securities available-for-sale [Abstract]
Amortized cost			6,892		8,252	8,530
Gross unrealized gains			70		1	1
Gross unrealized losses			43		93	102
Fair value			6,919		8,160	8,429
Mortgage-Backed Securities [Member]
Amortized cost, fair values, and unrealized gains and losses of securities available-for-sale [Abstract]
Amortized cost	[1]		270,667	[2]	720,562	421,263
Gross unrealized gains	[1]		4,243	[2]	17,359	18,050
Gross unrealized losses	[1]		5,341	[2]	48	39
Fair value	[1]		269,569	[2]	737,873	439,274
Securities Transferred [Abstract]
Fair value of securities transferred to held-to-maturity category		$ 316,900
Corporate Securities [Member]
Amortized cost, fair values, and unrealized gains and losses of securities available-for-sale [Abstract]
Amortized cost					45,010	25,002
Gross unrealized gains					927	159
Gross unrealized losses					18	98
Fair value					45,919	25,063
Other [Member]
Amortized cost, fair values, and unrealized gains and losses of securities available-for-sale [Abstract]
Amortized cost			46,513		492	428
Gross unrealized gains			0		0	0
Gross unrealized losses			0		0	0
Fair value			$ 46,513		$ 492	$ 428

[1] All Mortgage-Backed securities consist of securities collateralized by residential real estate and were issued by an agency or government-sponsored entity of the U.S. government.

[2] During Q1 2021, the Company transferred $316.9 million of AFS securities to HTM.