Components of Net Periodic Benefit Costs (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Retirement Benefits [Abstract]
Years of employment benefits are based upon			5 years
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined contribution plan, employer contribution amount	$ 12,000,000.0		$ 12,000,000.0
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	2,032,000	$ 1,746,000	5,906,000	$ 4,748,000
Interest cost	1,824,000	2,090,000	5,887,000	6,478,000
Expected return on plan assets	(5,908,000)	(5,802,000)	(17,383,000)	(15,256,000)
Amortization of Prior service cost	0	0	0	0
Amortization of Net loss	1,492,000	770,000	3,054,000	2,300,000
Net periodic (income) benefit cost	(560,000)	(1,196,000)	(2,536,000)	(1,730,000)
RIM Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	67,000	53,000	201,000	159,000
Interest cost	102,000	116,000	306,000	348,000
Expected return on plan assets	0	0	0	0
Amortization of Prior service cost	0	0	0	0
Amortization of Net loss	99,000	61,000	297,000	183,000
Net periodic (income) benefit cost	268,000	230,000	804,000	690,000
Post-retirement Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	99,000	84,000	297,000	252,000
Interest cost	171,000	207,000	513,000	621,000
Expected return on plan assets	0	0	0	0
Amortization of Prior service cost	0	0	0	0
Amortization of Net loss	77,000	37,000	231,000	111,000
Net periodic (income) benefit cost	347,000	328,000	1,041,000	984,000
Split-Dollar Life Insurance
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	120,000	88,000	346,000	264,000
Interest cost	135,000	114,000	384,000	342,000
Expected return on plan assets	0	0	0	0
Amortization of Prior service cost	14,000	14,000	42,000	42,000
Amortization of Net loss	113,000	62,000	339,000	186,000
Net periodic (income) benefit cost	382,000	$ 278,000	1,111,000	$ 834,000
Former Bank President | RIM Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic (income) benefit cost	$ 4,000,000		$ 8,000,000