Insurance Operations - Unpaid Losses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserve for unpaid losses and loss expenses, at beginning of year	$ 1,396,812	$ 1,258,237	$ 1,166,619
Ceded reserve for unpaid losses and loss expenses, at beginning of year	185,295	201,156	176,651
Net reserve for unpaid losses and loss expenses, at beginning of year	1,211,517	1,057,081	989,968
Incurred losses and loss expenses occurring in the:
Current year	482,989	439,847	373,423
Prior years	3,154	(5,017)	20,318
Prior years attributable to adjusted premium	14,882
Total net losses and loss adjustment expenses incurred	501,025	434,830	393,741
Paid losses and loss expenses for claims occurring in the:
Current year	66,522	47,734	54,026
Prior years	318,324	232,660	272,602
Total paid losses and loss expenses for claims	384,846	280,394	326,628
Net reserves for unpaid losses and loss expenses, at end of period	1,327,696	1,211,517	1,057,081
Ceded reserve for unpaid losses and loss expenses, at end of period	193,952	185,295	201,156
Gross reserve for unpaid losses and loss expenses, at end of period	1,521,648	1,396,812	1,258,237
Commercial Auto
Incurred losses and loss expenses occurring in the:
Prior years		(15,600)	33,200
General Liability
Incurred losses and loss expenses occurring in the:
Prior years	11,300	16,500
Workers' Compensation
Incurred losses and loss expenses occurring in the:
Prior years	(22,800)	(14,400)	$ (12,400)
Primary Workers Compensation line
Incurred losses and loss expenses occurring in the:
Prior years		6,200
Excess Workers Compensation line
Incurred losses and loss expenses occurring in the:
Prior years		8,200
Commercial Multiple Peril
Incurred losses and loss expenses occurring in the:
Prior years	$ 16,400	12,200
Commercial Multiple Peril | Media and Entertainment
Incurred losses and loss expenses occurring in the:
Prior years		12,200
Marine Liability
Incurred losses and loss expenses occurring in the:
Prior years		$ (4,100)