Reinsurance (Ceded credit risk) (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2018	Dec. 31, 2017
Ceded Credit Risk [Line Items]
Provision for uncollectible reinsurance relating to losses recoverable		$ 8,403	$ 8,848
Reinsurance Recoverable
Ceded Credit Risk [Line Items]
Reinsurance recoverable		$ 1,039,836	$ 1,280,870
Percentage of total		100.00%	100.00%
Reinsurance Recoverable | A- or better
Ceded Credit Risk [Line Items]
Reinsurance recoverable		$ 1,029,021	$ 1,270,503
Percentage of total		99.00%	99.20%
Reinsurance Recoverable | Top 10 reinsurers
Ceded Credit Risk [Line Items]
Reinsurance recoverable		$ 878,685	$ 1,055,445
Percentage of total		84.50%	82.50%
Reinsurance Recoverable | Top 10 reinsurers | Fully collateralized reinsurers | NR
Ceded Credit Risk [Line Items]
Reinsurance recoverable		$ 407,361	$ 459,339
Percentage of total		39.20%	35.90%
Reinsurance Recoverable | Top 10 reinsurers | Everest Re | A plus
Ceded Credit Risk [Line Items]
Reinsurance recoverable		$ 128,360	$ 128,206
Percentage of total		12.30%	10.00%
Reinsurance Recoverable | Top 10 reinsurers | Lloyds Syndicates | A plus
Ceded Credit Risk [Line Items]
Reinsurance recoverable		$ 69,885	$ 74,277
Percentage of total		6.70%	5.80%
Reinsurance Recoverable | Top 10 reinsurers | Swiss Re | AA- rating
Ceded Credit Risk [Line Items]
Reinsurance recoverable		$ 56,892	$ 65,218
Percentage of total		5.50%	5.10%
Reinsurance Recoverable | Top 10 reinsurers | Munich Re | AA- rating
Ceded Credit Risk [Line Items]
Reinsurance recoverable		$ 50,097	$ 94,180
Percentage of total		4.80%	7.40%
Reinsurance Recoverable | Top 10 reinsurers | Hannover Re | AA- rating
Ceded Credit Risk [Line Items]
Reinsurance recoverable		$ 49,501	$ 53,523
Percentage of total		4.80%	4.20%
Reinsurance Recoverable | Top 10 reinsurers | Transatlantic Re | A plus
Ceded Credit Risk [Line Items]
Reinsurance recoverable		$ 38,446	$ 33,729
Percentage of total		3.70%	2.60%
Reinsurance Recoverable | Top 10 reinsurers | Qatar Insurance Company | A rating
Ceded Credit Risk [Line Items]
Reinsurance recoverable		$ 30,981	$ 50,160
Percentage of total		3.00%	3.90%
Reinsurance Recoverable | Top 10 reinsurers | Markel | A rating
Ceded Credit Risk [Line Items]
Reinsurance recoverable		$ 25,893	$ 28,068
Percentage of total		2.50%	2.20%
Reinsurance Recoverable | Top 10 reinsurers | XL Catlin | A plus
Ceded Credit Risk [Line Items]
Reinsurance recoverable		$ 21,269
Percentage of total		2.00%
Reinsurance Recoverable | Top 10 reinsurers | Federal Crop Insurance Corporation
Ceded Credit Risk [Line Items]
Reinsurance recoverable	[1]		$ 68,745
Percentage of total	[1]		5.40%
Reinsurance Recoverable | Other reinsurers' balances greater than $1 million
Ceded Credit Risk [Line Items]
Reinsurance recoverable		$ 153,572	$ 218,226
Percentage of total		14.80%	17.00%
Reinsurance Recoverable | Other reinsurers' balances less than $1 million
Ceded Credit Risk [Line Items]
Reinsurance recoverable		$ 7,579	$ 7,199
Percentage of total		0.70%	0.50%

[1] The Company participates in a crop reinsurance program sponsored by the U.S. federal government. The Company remains obligated for amounts ceded in the event that its reinsurers or retrocessionaires do not meet their obligations, except for amounts ceded to the U.S. federal government in the Insurance segment agriculture line of business.