Disclosures About Fair Value of Financial Instruments - Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Significant unrealized appreciation or depreciation in financial instruments	$ 0	$ 0
Financial assets:
Securities available-for-sale	765,579,000	819,901,000
Securities held-to-maturity	17,968,000	18,223,000
Accrued interest receivable	25,531,000	25,755,000
Derivative assets	55,360,000	21,593,000
Financial liabilities:
Junior subordinated debentures	121,813,000	121,800,000
Interest rate swaps	55,496,000	20,991,000
Level 1
Financial assets:
Cash and cash equivalents	276,454,000	60,846,000
Securities available-for-sale	0	0
Securities held-to-maturity	0	0
Loans receivable, net	0	0
Residential mortgage loans held-for-sale	0	0
Accrued interest receivable	25,531,000	25,755,000
FHLB stock	0	0
Total financial assets	301,985,000	86,601,000
Financial liabilities:
Savings and checking deposits	7,298,979,000	7,022,597,000
Time deposits	0	0
Borrowed funds	194,224,000	246,341,000
Junior subordinated debentures	0	0
Accrued interest payable	834,000	1,142,000
Total financial liabilities	7,494,037,000	7,270,080,000
Level 2
Financial assets:
Cash and cash equivalents	0	0
Securities available-for-sale	765,579,000	819,901,000
Securities held-to-maturity	17,968,000	18,223,000
Loans receivable, net	0	0
Residential mortgage loans held-for-sale	0	0
Accrued interest receivable	0	0
FHLB stock	0	0
Total financial assets	838,400,000	859,158,000
Financial liabilities:
Savings and checking deposits	0	0
Time deposits	0	0
Borrowed funds	0	0
Junior subordinated debentures	0	0
Accrued interest payable	0	0
Total financial liabilities	55,496,000	20,991,000
Level 3
Financial assets:
Cash and cash equivalents	0	0
Securities available-for-sale	0	0
Securities held-to-maturity	0	0
Loans receivable, net	8,751,479,000	8,666,149,000
Residential mortgage loans held-for-sale	6,426,000	7,709,000
Accrued interest receivable	0	0
FHLB stock	0	0
Total financial assets	8,758,412,000	8,674,417,000
Financial liabilities:
Savings and checking deposits	0	0
Time deposits	1,514,631,000	1,574,063,000
Borrowed funds	0	0
Junior subordinated debentures	117,557,000	115,518,000
Accrued interest payable	0	0
Total financial liabilities	1,632,188,000	1,689,581,000
Carrying amount
Financial assets:
Cash and cash equivalents	276,454,000	60,846,000
Securities available-for-sale	765,579,000	819,901,000
Securities held-to-maturity	17,208,000	18,036,000
Loans receivable, net	8,737,551,000	8,743,024,000
Residential mortgage loans held-for-sale	6,426,000	7,709,000
Accrued interest receivable	25,531,000	25,755,000
FHLB stock	13,131,000	14,740,000
Total financial assets	9,897,240,000	9,711,604,000
Financial liabilities:
Savings and checking deposits	7,298,979,000	7,022,597,000
Time deposits	1,493,756,000	1,569,410,000
Borrowed funds	191,599,000	246,336,000
Junior subordinated debentures	121,813,000	121,800,000
Accrued interest payable	834,000	1,142,000
Total financial liabilities	9,162,477,000	8,982,276,000
Estimated fair value
Financial assets:
Cash and cash equivalents	276,454,000	60,846,000
Securities available-for-sale	765,579,000	819,901,000
Securities held-to-maturity	17,968,000	18,223,000
Loans receivable, net	8,751,479,000	8,666,149,000
Residential mortgage loans held-for-sale	6,426,000	7,709,000
Accrued interest receivable	25,531,000	25,755,000
FHLB stock	13,131,000	14,740,000
Total financial assets	9,911,928,000	9,634,916,000
Financial liabilities:
Savings and checking deposits	7,298,979,000	7,022,597,000
Time deposits	1,514,631,000	1,574,063,000
Borrowed funds	194,224,000	246,341,000
Junior subordinated debentures	117,557,000	115,518,000
Accrued interest payable	834,000	1,142,000
Total financial liabilities	9,181,721,000	8,980,652,000
Interest rate lock commitments | Level 1
Financial assets:
Derivative assets	0	0
Interest rate lock commitments | Level 2
Financial assets:
Derivative assets	0	0
Interest rate lock commitments | Level 3
Financial assets:
Derivative assets	507,000	559,000
Interest rate lock commitments | Carrying amount
Financial assets:
Derivative assets	507,000	559,000
Interest rate lock commitments | Estimated fair value
Financial assets:
Derivative assets	507,000	559,000
Forward commitments | Level 1
Financial assets:
Derivative assets	0	0
Forward commitments | Level 2
Financial assets:
Derivative assets	197,000	145,000
Forward commitments | Level 3
Financial assets:
Derivative assets	0	0
Forward commitments | Carrying amount
Financial assets:
Derivative assets	197,000	145,000
Forward commitments | Estimated fair value
Financial assets:
Derivative assets	197,000	145,000
Interest rate swaps | Level 1
Financial assets:
Derivative assets	0	0
Financial liabilities:
Interest rate swaps	0	0
Interest rate swaps | Level 2
Financial assets:
Derivative assets	54,656,000	20,889,000
Financial liabilities:
Interest rate swaps	55,376,000	20,952,000
Interest rate swaps | Level 3
Financial assets:
Derivative assets	0	0
Financial liabilities:
Interest rate swaps	0	0
Interest rate swaps | Carrying amount
Financial assets:
Derivative assets	54,656,000	20,889,000
Financial liabilities:
Interest rate swaps	55,376,000	20,952,000
Interest rate swaps | Estimated fair value
Financial assets:
Derivative assets	54,656,000	20,889,000
Financial liabilities:
Interest rate swaps	55,376,000	20,952,000
Risk participation agreements | Level 1
Financial liabilities:
Interest rate swaps	0	0
Risk participation agreements | Level 2
Financial liabilities:
Interest rate swaps	120,000	39,000
Risk participation agreements | Level 3
Financial liabilities:
Interest rate swaps	0	0
Risk participation agreements | Carrying amount
Financial liabilities:
Interest rate swaps	120,000	39,000
Risk participation agreements | Estimated fair value
Financial liabilities:
Interest rate swaps	$ 120,000	$ 39,000