Investment Securities and Equity Securities with Readily Determinable Fair Values (Contractual Maturities and Estimated Fair Values) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Amortized Cost, Rolling Maturity [Abstract]
Due in one year or less, held-to-maturity debt securities amortized cost	$ 1,075,000
Due after one year through five years, held-to-maturity debt securities amortized cost	2,325,000
Amortized cost, held-to-maturity debt securities	3,400,000		$ 3,400,000
Debt Securities, Held-to-maturity, Maturity, Fair Value, Rolling Maturity [Abstract]
Due in one year or less, held-to-maturity debt securities, Estimated fair value	1,075,000
Due after one year through five years, held-to-maturity debt securities, Estimated fair value	2,325,000
Estimated fair value	3,400,000		3,400,000
Available-for-sale debt securities amortized cost disclosures
Due in one year or lesss, available-for-sale debt securities amortized cost	945,682,000
Due after five years through ten years, available-for-sale debt securities amortized cost	355,000
Due after ten years, available-for-sale debt securities amortized cost	44,756,000
Residential mortgage-backed securities, amortized cost	3,043,953,000
Amortized cost, Available-for-sale securities	4,034,746,000		3,054,289,000
Available for sale debt securities, Estimated Fair Value Disclosures
Due in one year or less, available-for-sale debt securities, Estimated Fair Value	945,682,000
Due after five years through ten years, available-for-sale debt securities, Estimated Fair Value	356,000
Due after ten years, available-for-sale debt securities, Estimated Fair Value	48,283,000
Residential mortgage-backed securities, Estimated Fair Value	3,063,036,000
Estimated fair value, Available for sale securities	4,057,357,000		$ 3,080,768,000
Available for sale debt securities, Estimated Fair Value Disclosures
Proceeds from sales and calls of available for sale securities	2,510,000	$ 18,920,000
Proceeds from sales of mortgage-backed securities	0	0
Gross gains realized on sales	0	0
Gross losses realized on sales	(4)	$ (5)
Collateral Pledged
Available-for-sale debt securities amortized cost disclosures
Amortized cost, Available-for-sale securities	1,273,037,000
Available for sale debt securities, Estimated Fair Value Disclosures
Fair value of available for sale investment securities pledged	$ 1,274,911,000