Reserves for Unpaid Losses and Loss Adjustment Expenses (Incurred and Paid Claims Development - Aviation) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) claim	Dec. 31, 2018 USD ($)	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 ($)	Dec. 31, 2010 USD ($)
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable net prior year development	$ 60,900	$ 5,953
Incurred claims and claim adjustment expenses related to current year	301,265	250,075
Total incurred	362,165	256,028
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	192,335	232,345
Paid claims and allocated claim adjustment expenses related to current year	127,610	90,640
Total paid	319,945	322,985
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	347,751	305,531	$ 372,488
Aviation [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total incurred	56,791
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Total paid	54,277
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	2,514
Aviation [Member] | Loss Adjustment Expenses Recognized In 2010 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable net prior year development		9,839	9,820	$ 9,489	$ 9,432	$ 9,316	$ 9,186	$ 9,570	$ 10,960	$ 11,331
Incurred claims and claim adjustment expenses related to current year	$ 9,839
Cumulative number of reported claims | claim	269
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 9,618	9,836	9,811	9,245	9,083	9,083	9,031	8,635	7,041	$ 5,613
Aviation [Member] | Loss Adjustment Expenses Recognized In 2011 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable net prior year development		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	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 net prior year development		9,941	10,021	10,024	10,336	10,353	10,738	10,988
Incurred claims and claim adjustment expenses related to current year	$ 9,941
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	$ 9,941	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 net prior year development		10,649	10,057	9,563	10,295	11,304	10,236
Incurred claims and claim adjustment expenses related to current year	$ 10,252
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	$ 10,242	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 net prior year development		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	3,565	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 net prior year development		1,631	1,643	1,709	1,870
Incurred claims and claim adjustment expenses related to current year	$ 1,630
Cumulative number of reported claims | claim	197
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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 net prior year development		2,119	2,241	2,330
Incurred claims and claim adjustment expenses related to current year	2,219
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 33
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	$ 2,082	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 net prior year development		2,082	2,325
Incurred claims and claim adjustment expenses related to current year	2,079
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 26
Cumulative number of reported claims | claim	319
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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 net prior year development		2,382
Incurred claims and claim adjustment expenses related to current year	1,990
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 143
Cumulative number of reported claims | claim	332
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 2,237	$ 1,716
Aviation [Member] | Loss Adjustment Expenses Recognized In 2019 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Incurred claims and claim adjustment expenses related to current year	5,246
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 836
Cumulative number of reported claims | claim	265
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to current year	$ 2,911