Acquisitions - Loss on Settlement Of Contractual Preexisting Relationships (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	May 14, 2018	Dec. 31, 2017	Dec. 31, 2016
ASSETS
Funds held by reinsured companies	$ 408,735	$ 321,267
Deferred acquisition costs/Value of business acquired	272,462	86,585		$ 80,192	$ 94,551
TOTAL ASSETS	19,363,315	16,556,270
LIABILITIES
Losses and LAE	10,429,238	9,409,504		$ 7,398,088	$ 5,987,867
Unearned premiums	746,775	842,618
Insurance and reinsurance balances payable	373,180	388,086
Other liabilities	464,014	468,129
TOTAL LIABILITIES	14,068,173	12,183,738
NET ASSETS (LIABILITIES)	$ 4,856,351	$ 3,913,989
Carrying value | KaylaRe
ASSETS
Funds held by reinsured companies			$ 386,793
Deferred acquisition costs/Value of business acquired			33,549
TOTAL ASSETS			420,342
LIABILITIES
Losses and LAE			339,747
Unearned premiums			105,602
Insurance and reinsurance balances payable			25,897
Other liabilities			1,864
TOTAL LIABILITIES			473,110
NET ASSETS (LIABILITIES)			(52,768)
Fair value | KaylaRe
ASSETS
Funds held by reinsured companies			386,793
Deferred acquisition costs/Value of business acquired			40,268
TOTAL ASSETS			427,061
LIABILITIES
Losses and LAE			333,205
Unearned premiums			105,602
Insurance and reinsurance balances payable			23,559
Other liabilities			1,864
TOTAL LIABILITIES			464,230
NET ASSETS (LIABILITIES)			(37,169)
Loss on deemed settlement | KaylaRe
ASSETS
Funds held by reinsured companies			0
Deferred acquisition costs/Value of business acquired			6,719
TOTAL ASSETS			6,719
LIABILITIES
Losses and LAE			(6,542)
Unearned premiums			0
Insurance and reinsurance balances payable			(2,338)
Other liabilities			0
TOTAL LIABILITIES			(8,880)
NET ASSETS (LIABILITIES)			$ 15,599