Comprehensive Income (Components of Comprehensive Income-Gross and Net of Tax) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net income, Gross	$ 85,257	$ 71,178	$ 143,947	$ 235,097
Unrealized holding gains during the period, Gross	37,836	34,392	132,623	220,864
Non-Credit Portion of OTTI Recognized in OCI; Gross	9,239		(18,672)
Amounts reclassified into net income: HTM securities, Gross	(1)	(3)	(7)	(33)
Realized (gains) losses on disposals and intent-to-sell OTTI on AFS securities, Gross	(515)	2,821	9,126	444
Credit loss (benefit) expense recognized in OTTI, Gross	(2,853)		8,010
Total unrealized gains on investment securities, Gross	43,706	37,210	131,080	221,275
Amount reclassified into net income: Net actuarial loss, Gross	754	665	2,261	1,993
Total defined benefit pension and post-retirement benefit plans, Gross	754	665	2,261	1,993
Other comprehensive income, Gross	44,460	37,875	133,341	223,268
Comprehensive income before tax	129,717	109,053	277,288	458,365
Net income, Tax	15,382	15,028	24,653	45,333
Unrealized holding gains during the period, Tax	7,947	7,224	27,852	46,381
Non-Credit Portion of OTTI Recognized in OCI; Tax	1,940		(3,921)
Amounts reclassified into net income: HTM securities, Tax	(1)	(1)	(2)	(7)
Realized (gains) losses on disposals and intent-to-sell OTTI on AFS securities, Tax	(109)	592	1,916	93
Credit loss (benefit) expense recognized in OTTI, Tax	(599)		1,682
Total unrealized gains on investment securities, Tax	9,178	7,815	27,527	46,467
Amount reclassified into net income: Net actuarial loss, Tax	158	140	474	419
Total defined benefit pension and post-retirement benefit plans, Tax	158	140	474	419
Other comprehensive income, Tax	9,336	7,955	28,001	46,886
Comprehensive income, Tax	24,718	22,983	52,654	92,219
Net income	69,875	56,150	119,294	189,764
Unrealized holding gains arising during period, Net	29,889	27,168	104,771	174,483
Non-Credit Portion of OTTI Recognized in OCI, Net	7,299	0	(14,751)	0
Amounts reclassified into net income: HTM Securities, Net	0	(2)	(5)	(26)
Realized (gains) losses on disposals and intent-to-sell OTTI on AFS Securities, Net	(406)	2,229	7,210	351
Credit loss (benefit) expense recognized in OTTI, Net	(2,254)	0	6,328	0
Total Unrealized Gains Losses on Investment Securities, Net	34,528	29,395	103,553	174,808
Amount reclassified into net income: Net actuarial loss, Net	596	525	1,787	1,574
Total defined benefit pension and post-retirement plans, Net	596	525	1,787	1,574
Other Comprehensive Income, Net	35,124	29,920	105,340	176,382
Comprehensive Income, Net	$ 104,999	$ 86,070	$ 224,634	$ 366,146