Reserves for Losses and Loss Adjustment Expenses (Incurred and Paid Claims Development - Aviation) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) claim	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development	$ 58,288	$ 60,900
Incurred claims and claim adjustment expenses related to current year	332,863	301,265
Total incurred	412,851	362,165
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	185,407	192,335
Paid claims and allocated claim adjustment expenses related to current year	113,312	127,610
Total paid	320,419	319,945
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	440,183	347,751	$ 305,531
Loss Adjustment Expenses Recognized In 2011 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years										$ 2,897
Aviation [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total incurred	49,948
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Total paid	47,501
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	2,448
Aviation [Member] | Loss Adjustment Expenses Recognized In 2011 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		10,028	10,028	$ 10,045	$ 9,884	$ 9,829	$ 9,729	$ 9,759	$ 11,299	12,330
Incurred claims and claim adjustment expenses related to current year	$ 10,028
Cumulative number of reported claims | claim	305
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 10,028	10,028	10,028	10,028	9,628	9,546	9,311	8,924	8,894	$ 6,313
Aviation [Member] | Loss Adjustment Expenses Recognized In 2012 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		9,941	9,941	10,021	10,024	10,336	10,353	10,738	10,988
Incurred claims and claim adjustment expenses related to current year	$ 9,446
Cumulative number of reported claims | claim	229
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 9,446	9,941	10,041	10,041	10,041	10,049	9,672	8,486	$ 5,641
Aviation [Member] | Loss Adjustment Expenses Recognized In 2013 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		10,252	10,649	10,057	9,563	10,295	11,304	10,236
Incurred claims and claim adjustment expenses related to current year	$ 10,297
Cumulative number of reported claims | claim	231
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 10,281	10,242	10,456	9,944	9,356	9,584	9,493	$ 6,537
Aviation [Member] | Loss Adjustment Expenses Recognized In 2014 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		3,567	3,566	3,558	3,627	3,654	3,179
Incurred claims and claim adjustment expenses related to current year	$ 3,567
Cumulative number of reported claims | claim	201
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 3,567	3,567	3,565	3,327	3,259	3,105	$ 2,779
Aviation [Member] | Loss Adjustment Expenses Recognized in 2015 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		1,630	1,631	1,643	1,709	1,870
Incurred claims and claim adjustment expenses related to current year	1,635
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 1
Cumulative number of reported claims | claim	198
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 1,634	1,630	1,601	1,520	1,405	$ 958
Aviation [Member] | Loss Adjustment Expenses Recognized in 2016 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		2,219	2,119	2,241	2,330
Incurred claims and claim adjustment expenses related to current year	2,220
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 4
Cumulative number of reported claims | claim	292
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 2,216	2,082	1,918	1,907	$ 1,469
Aviation [Member] | Loss Adjustment Expenses Recognized in 2017 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		2,079	2,082	2,325
Incurred claims and claim adjustment expenses related to current year	2,178
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 11
Cumulative number of reported claims | claim	320
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 2,054	2,021	1,837	$ 1,260
Aviation [Member] | Loss Adjustment Expenses Recognized In 2018 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		1,990	2,382
Incurred claims and claim adjustment expenses related to current year	2,565
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 49
Cumulative number of reported claims | claim	336
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 2,368	2,237	$ 1,716
Aviation [Member] | Loss Adjustment Expenses Recognized In 2019 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		5,246
Incurred claims and claim adjustment expenses related to current year	4,525
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 119
Cumulative number of reported claims | claim	290
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 3,787	$ 2,911
Aviation [Member] | Loss Adjustment Expenses Recognized In 2020 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Incurred claims and claim adjustment expenses related to current year	3,487
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 884
Cumulative number of reported claims | claim	270
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to current year	$ 2,120