Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	$ 6,097,973	$ 6,097,973	$ 5,879,986
Allowance for credit losses, Period beginning balance	10,395	0
Allowance for credit losses, ending balance at September 30, 2020	7,180	7,180
Interest and dividends due or accrued	46,392	46,392	44,846
Current provision for securities without prior allowance	1,025	8,100
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities	(3,856)	0
Reductions for securities sold	(362)	(830)
Reductions for securities identified as intent (or requirement) to sell during the period	(22)	(90)
Unrealized Gains	371,159	371,159	223,372
Unrealized Losses	(24,415)	(24,415)	(7,738)
Debt Securities, Available-for-sale, Fair Value	6,437,537	6,437,537	6,095,620
Available-for-sale Securities [Member]
Debt Securities, Available-for-sale [Line Items]
AFS Unrealized Losses	24,400	24,400	7,700
Available-for-sale Securities | U.S. Government and Government Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	110,201	110,201	112,680
Allowance for credit losses, ending balance at September 30, 2020	0	0
Unrealized Gains	7,374	7,374	3,506
Unrealized Losses	(46)	(46)	0
Debt Securities, Available-for-sale, Fair Value	117,529	117,529	116,186
Available-for-sale Securities | Foreign Government [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	15,679	15,679	18,011
Allowance for credit losses, Period beginning balance	28	0
Allowance for credit losses, ending balance at September 30, 2020	7	7
Current provision for securities without prior allowance	0	26
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities	(2)	0
Reductions for securities sold	(19)	(19)
Reductions for securities identified as intent (or requirement) to sell during the period	0	0
Unrealized Gains	1,336	1,336	533
Unrealized Losses	0	0	(2)
Debt Securities, Available-for-sale, Fair Value	17,008	17,008	18,542
Available-for-sale Securities | Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	1,152,794	1,152,794	1,168,185
Allowance for credit losses, Period beginning balance	17	0
Allowance for credit losses, ending balance at September 30, 2020	15	15
Current provision for securities without prior allowance	9	15
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities	(11)	0
Reductions for securities sold	0	0
Reductions for securities identified as intent (or requirement) to sell during the period	0	0
Unrealized Gains	78,841	78,841	62,175
Unrealized Losses	(164)	(164)	(270)
Debt Securities, Available-for-sale, Fair Value	1,231,456	1,231,456	1,230,090
Available-for-sale Securities | Corporate Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	2,154,302	2,154,302	1,866,881
Allowance for credit losses, Period beginning balance	8,077	0
Allowance for credit losses, ending balance at September 30, 2020	5,358	5,358
Current provision for securities without prior allowance	1,016	6,100
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities	(3,455)	0
Reductions for securities sold	(265)	(659)
Reductions for securities identified as intent (or requirement) to sell during the period	(15)	(83)
Unrealized Gains	157,689	157,689	81,906
Unrealized Losses	(7,802)	(7,802)	(1,310)
Debt Securities, Available-for-sale, Fair Value	2,298,831	2,298,831	1,947,477
Available-for-sale Securities | Collateralized Loan Obligations and Other Asset Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	942,627	942,627	790,517
Allowance for credit losses, Period beginning balance	1,389	0
Allowance for credit losses, ending balance at September 30, 2020	1,121	1,121
Current provision for securities without prior allowance	0	1,237
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities	(210)	0
Reductions for securities sold	(51)	(109)
Reductions for securities identified as intent (or requirement) to sell during the period	(7)	(7)
Unrealized Gains	18,274	18,274	7,929
Unrealized Losses	(14,198)	(14,198)	(5,434)
Debt Securities, Available-for-sale, Fair Value	945,582	945,582	793,012
Available-for-sale Securities | Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	1,122,678	1,122,678	1,409,003
Allowance for credit losses, Period beginning balance	831	0
Allowance for credit losses, ending balance at September 30, 2020	647	647
Current provision for securities without prior allowance	0	690
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities	(157)	0
Reductions for securities sold	(27)	(43)
Reductions for securities identified as intent (or requirement) to sell during the period	0	0
Unrealized Gains	61,259	61,259	43,421
Unrealized Losses	(333)	(333)	(455)
Debt Securities, Available-for-sale, Fair Value	1,182,957	1,182,957	1,451,969
Available-for-sale Securities | Commercial Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost/Amortized Cost	599,692	599,692	514,709
Allowance for credit losses, Period beginning balance	53	0
Allowance for credit losses, ending balance at September 30, 2020	32	32
Current provision for securities without prior allowance	0	32
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities	(21)	0
Reductions for securities sold	0	0
Reductions for securities identified as intent (or requirement) to sell during the period	0	0
Unrealized Gains	46,386	46,386	23,902
Unrealized Losses	(1,872)	(1,872)	(267)
Debt Securities, Available-for-sale, Fair Value	644,174	644,174	$ 538,344
Available-for-sale Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Interest and dividends due or accrued	45,300	45,300
Current provision for securities without prior allowance	2,854	(8,010)
Available-for-sale Securities [Member] | U.S. Government and Government Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Current provision for securities without prior allowance		0
Available-for-sale Securities [Member] | Foreign Government [Member]
Debt Securities, Available-for-sale [Line Items]
Current provision for securities without prior allowance	3	(25)
Available-for-sale Securities [Member] | Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Current provision for securities without prior allowance	2	(15)
Available-for-sale Securities [Member] | Corporate Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Current provision for securities without prior allowance	2,454	(6,018)
Available-for-sale Securities [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Current provision for securities without prior allowance	217	(1,230)
Available-for-sale Securities [Member] | Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Current provision for securities without prior allowance	157	(690)
Available-for-sale Securities [Member] | Commercial Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Current provision for securities without prior allowance	$ 21	$ (32)