Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share, Diluted
Net income attributable to NGHC	$ 109,969	$ 71,154	$ 377,302	$ 240,788
Dividends on preferred stock	(7,875)	(7,875)	(24,675)	(24,675)
Numerator for basic EPS	102,094	63,279	352,627	216,113
Numerator for diluted EPS - after assumed conversions	$ 102,094	$ 63,279	$ 353,677	$ 217,163
Denominator for basic EPS - weighted-average shares outstanding	113,418,411	113,263,367	113,505,785	113,153,121
Convertible preferred stock	789,473	789,473	789,473	789,473
Dilutive potential common shares	3,247,963	2,875,122	2,726,067	2,934,403
Denominator for diluted EPS - weighted-average shares outstanding and assumed conversions	116,666,374	116,138,489	116,231,852	116,087,524
Earnings Per Share, Basic
Basic EPS	$ 0.90	$ 0.56	$ 3.11	$ 1.91
Earnings Per Share, Diluted
Diluted EPS	$ 0.88	$ 0.54	$ 3.04	$ 1.87
Employee stock options
Earnings Per Share, Diluted
Effect of dilutive securities	1,924,602	1,823,012	1,654,114	1,881,295
RSUs
Earnings Per Share, Diluted
Effect of dilutive securities	533,888	262,637	282,480	263,635
Preferred stock dividends - nonconvertible
Earnings Per Share, Diluted
Dividends on preferred stock	$ (7,875)	$ (7,875)	$ (23,625)	$ (23,625)
Preferred stock dividends - convertible
Earnings Per Share, Diluted
Dividends on preferred stock	0	0	(1,050)	(1,050)
Preferred stock dividends - convertible	$ 0	$ 0	$ 1,050	$ 1,050