Income Taxes - Schedule of Reconciliation of Income Tax Expense Attributable to Income from Operations with Amounts Computed by Applying U.S. Federal Tax Rate to Pretax Income from Continuing Operations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Reconciliation of income tax expense reported to amount computed by applying the U.S. federal tax rate
Provision for income taxes at the statutory rate	$ 21,424	$ 24,124	$ 40,290	$ 7,458
Increase (reduction) in taxes resulting from:
Excess tax benefit on share-based compensation	(270)	(265)	(2,194)	(1,293)
Tax exempt interest income	(310)	(323)	(629)	(636)
Dividends received deduction	(248)	(212)	(484)	(484)
Investment tax credit	(801)	(129)	(1,603)	(1,449)
ESOP dividends paid deduction	(137)	(137)	(267)	(269)
Nondeductible expenses	826	161	1,237	364
Other items, net	(278)	(506)	678	925
Total tax expense	$ 20,206	$ 22,713	$ 37,028	$ 4,616
Reconciliation of income tax expense rate to the U.S. federal tax rate
Tax rate used (as a percent)	21.00%	21.00%	21.00%	21.00%
Excess tax benefit on share-based compensation (as a percent)	(0.30%)	(0.20%)	(1.10%)	(3.60%)
Effective rate reduction due to tax exempt interest income (as a percent)	(0.30%)	(0.30%)	(0.30%)	(1.80%)
Effective rate reduction due to dividend received (as a percent)	(0.20%)	(0.20%)	(0.30%)	(1.40%)
Effective rate reduction due to investment tax credit (as a percent)	(0.80%)	(0.10%)	(0.80%)	(4.00%)
Effective rate reduction due to dividend paid to ESOP (as a percent)	(0.10%)	(0.10%)	(0.10%)	(0.80%)
Effective rate reduction due to nondeductible expenses (as a percent)	0.80%	0.10%	0.60%	1.00%
Effective rate reduction due to other items, net (as a percent)	(0.30%)	(0.40%)	0.30%	2.60%
Total tax expense	19.80%	19.80%	19.30%	13.00%